9 March 1928:
Off Yokosuka Bay. I-21 is conducting speed trials. About 1100, I-21 collides with destroyer SHIOKAZE on torpedo launch trials. Both ships suffer light damage, but no casualties. I-21 returns to Yokosuka to obtain repairs to her bow - bent 60 degrees to starboard.
22 March 1938:
Shantung province, NE China. I-21 and I-22 land a force of 30 Shanghai Special Naval Landing Force (SNLF) troops on Liugong (Liukung) Island in Weihai Bay.
1 June 1938:
I-21 is renumbered I-121.
1 May 1941:
I-121 is assigned to the Third Fleet in Rear Admiral Kono Chimakiís SubRon 6 under Captain (Rear Admiral posthumously) Miyazaki Takejiís (46)(former of RO-57, I-122, I-1) SubDiv 13 with I-122.
I-121, under LtCdr (later Captain) Endo Shinobu (52), departs Yokosuka for Samah, Hainan Island, China.
1 December 1941:
2 December 1941:
The coded signal "Niitakayama nobore (Climb Mt. Niitaka) 1208" is received from the Combined Fleet. It signifies that hostilities will commence on 8 December (Japan time). Mt. Niitaka, located in Formosa (now Taiwan), is then the highest point in the Japanese Empire.
6 December 1941: Operation "M" - The Attack on the Southern Philippines:
Vice Admiral Takahashi Iboís (35)(former CO of KIRISHIMA) Second Fleet, Southern Force, Philippines Seizure Force departs Palau.
7 December 1941:
I-121 and I-122 lay two minefields NE of Singapore. I-121 lays 42 Type 88 mines in the approximate area of 0-40N, 115E and then commences her first war patrol off the eastern entrance of Johore Strait.
11 December 1941:
Vice Admiral Takahashi's force invades Legaspi covered by light carrier RYUJO, CruDiv 5's HAGURO, MYOKO, NACHI, DesRon 2's light cruiser JINTSU and nine destroyers and Rear Admiral (later Vice Admiral) Kubo Kyujiís (35)(former CO of MIKUMA) seaplane tenders CHITOSE, MIZUHO, light cruisers NAKA, NAGARA, five destroyers and seven transports.
19-20 December 1941:
Takahashi's force invades Davao, Philippines.
24 December 1941:
Takahashi's force invades Jolo, Philippines.
11 January 1942:
Timor Sea, SW of Tiwi Islands. At 0530, Cdr Endo attacks an unidentified Allied destroyer.
12 January 1942:
Clarence Strait. By 0455, I-121 lays 39 mines and then returns to her former station in Timor Sea.
14 January 1942:
I-124 sights USS HOUSTON (CA-30) and two destroyers but is unable to attack.
18 January 1942:
Timor Sea, N of Wetar Island. I-121 chases an unidentified Allied merchant escorted by a small patrol vessel and attacks it with three torpedoes. Following this attack, I-121 returns to Clarence Strait. 
21 January 1942:
Clarence Strait, off Darwin. I-121 is chased by several patrol vessels, dropping a total of 42 depth charges. She receives medium damage to two fuel tanks, but escapes. 
25 January 1942:
Departs the patrol area, heading for the scheduled rendezvous area with I-124.
30 January 1942:
After the loss of I-124, SubRon 6's I-121 and I-122 arrive at Davao, Philippines. They are serviced by tender CHOGEI. I-121's damaged fuel tanks are repaired.
1 February 1942:
LtCdr (later Cdr) Fujimori Yasuo (56)(former CO of RO-60) assumes command.
9 February 1942:
Departs Davao in company of I-122 on her third war patrol to reconnoiter Port Darwin prior to the attack of Vice Admiral (Admiral, posthumously) Nagumo Chuichi's (36) Striking Force and transmit weather reports.
I-121 and I-122 depart Davao. I-121 is to act as a weather ship in the Arafura Sea. She patrols 300 miles NE of Morotai.
13 February 1942:
I-122 detaches and proceeds SE. I-121 continues south.
16 February 1942:
By sundown I-121 arrives at her assigned area 50 miles NW of Darwin.
18 February 1942:
I-21 surfaces on station prior to sunset to recharge her batteries, but is strafed by an Australian patrol plane. Fujimori crash-dives. After dark he resurfaces and at 2030 sends a weather report that is relayed to Vice Admiral Nagumoís Strike Force then enroute from Palau.
19 February 1942: The Attack on Darwin:
At 0230 Fujimori sends a second weather report. At 0957 (local), Nagumo's Striking Force raids Port Darwin. Seventy-one "Kate" attack planes, 81 "Val" dive-bombers and 36 "Zeke" fighters led by Cdr (later Captain) Fuchida Minoru (of Pearl Harbor) from CarDiv 1's AKAGI and KAGA and CarDiv 2's HIRYU and SORYU attack Darwin.
They destroy 15 aircraft including nine American Curtiss P-40 "Kittyhawk" fighters, sink eight ships including destroyer USS PEARY (DD-226) and large Army transport GENERAL M.C. MEIGS, damage nine ships including seaplane tender (WW1 destroyer conversion) USS WILLIAM B. PRESTON (AVD-7).
The carrier strike is followed by a strike of 28 twin engine land-based Mitsubishi G3M2 "Nells" of the 1st Kokutai from Ambon and 27 Mitsubishi G4M1 "Bettys" of the Kanoya Kokutai based at Kendari, Celebes.25 February 1942:
10 March 1942:
I-121 and -122 depart Staring Bay for Japan. On that same day, both submarines are directly attached to the Combined Fleet HQ.
16 March 1942:
Vice Admiral, the Marquis, Komatsu Teruhisaís (37)(former CO of CA NACHI) assumes command of the Sixth Fleet (Submarines).
21 March 1942:
Arrives at Kure for repairs.
8 May 1942: Operation "MI": The Battle of Midway:
Departs Kure for Kwajalein
21 May 1942:
Departs Kwajalein for the French Frigate Shoals to participate in Operation K-2.
30 May 1942:
I-121 receives I-123's message about American vessels sighted at French Frigate Shoals.
1 June 1942:
Departs French Frigate Shoals and heads west.
4 June 1942:
I-121 and I-123 receive the order to proceed westward.
5 June 1942:
SW of Lisianski Island. I-121 briefly sights a surfaced American submarine, heading NE, but fails to reach the firing position. 
25 June 1942:
Arrives at Kwajalein in company of I-122 and I-123, later returns to Yokosuka.
14 July 1942:
Capt Miyazaki Takeji's SubDiv 13 is reassigned to SubRon 7, Eighth Fleet.
16 July 1942:
Departs Yokosuka for Truk in company of I-122.
4 August 1942:
Arrives at Rabaul.
7 August 1942 - American Operation "Watchtower" - The Invasion of Guadalcanal, Solomons:
Rear Admiral (later Admiral) Richmond K. Turner's (USNA í04) Amphibious Task Force 62, covered by Vice Admiral (later Admiral) Frank J. Fletcher's (USNA í06) Task Force 61 and Rear Admiral (later Admiral) John S. McCain's (USNA í06) Task Force 63's land-based aircraft, lands Maj Gen (later General/Commandant) Alexander A. Vandegrift's 1st Marine Division on Florida, Tulagi, Gavutu, Tanambogo and Guadalcanal opening a seven-month campaign to take the island.
That day, I-121 and I-122 depart Rabaul to reconnoiter Guadalcanal and Tulagi.
8 August 1942:
30 miles S of Cape St. George, New Ireland. Lookouts aboard LtCdr (later Captain) Henry G. Munsonís (USNA í32) USS S-38 spot the I-121 enroute on the surface from Rabaul to Savo Island, but Munson is unable to attack.
22 August 1942:
50 miles SE of San Cristobal. At 1710, I-121 attacks Rear Admiral Thomas C. Kinkaid's Task Force 16. One torpedo passes between USS ENTERPRISE (CV-6) and heavy cruiser USS PORTLAND (CA-33), broaching briefly.
24 August 1942: The Battle of the Eastern Solomons:
Vice Admiral (later Admiral) Frank J. Fletcherís Task Force 61's USS SARATOGA (CV-3) and ENTERPRISE launch aircraft that find and sink light carrier RYUJO. In turn, CarDiv 1ís SHOKAKU and ZUIKAKU launch aircraft that find and damage ENTERPRISE. That evening, aircraft from SARATOGA damage the seaplane carrier CHITOSE.
29 August 1942:
80 miles NE of San Cristobal. At 0430, I-121 sights one carrier and several destroyers; at 0800 she sights one carrier, two cruisers and four destroyers.
8 September 1942:
20 September 1942:
Returns to Kure for an overhaul.
15 October 1942:
Lt Shimada Takeo (59)(former RO-62) is appointed the Commanding Officer.
16 November 1942: Vice Admiral Komatsu convenes a meeting of his submarine captains at Truk. He announces that the Sixth Fleet has been ordered by Admiral (Fleet Admiral posthumously) Yamamoto Isoroku (32), CINC, Combined Fleet, to organize a supply system for the IJA's 17th Army garrison on Guadalcanal.
1 December 1942:
Departs Kure for Truk.
21 December 1942:
Arrives at Rabaul. Reassigned to Submarine Group B.
23 December 1942:
Departs Rabaul on her first supply run to Buna, eastern New Guinea.
14 February 1943:
Departs Rabaul for Truk.
5 March 1943:
Arrives at Kure for overhaul.
25 April 1943:
7 May 1943:
Arrives at Rabaul.
10 May - August 1943:
Departs Rabaul on her first supply run to Lae, New Guinea.
19 August 1943:
20 August 1943:
Arrives at Lae on her tenth supply run there. During all ten supply runs to Lae I-121 delivers 264 tons of cargo, evacuating a total of 148 men. On that same day, Lt Watanabe Masaki (63) is appointed the Commanding Officer.
1 September 1943:
Returns to Kure. I-121 and I-122 are withdrawn from combat and assigned to training duties at the Kure Naval Base thereafter.
Kure Navy Yard. The Submarine School carries out Sensuikan Gaigen Toshoku Jikken experiments on camouflage painting of I-boats based on RO-500's (ex-German U-boat U 511) camouflage. I-121 is painted 5-Go shoku.
5 January 1944:
Iyo Nada. On this day, an experiment is carried to determine the camouflage's horizontal visibility, its visibility from aircraft, the appropriateness of the color for the surrounding sea area, its ability to confuse determining the I-boat's speed and direction and the durability of the paint. The experiment is deemed a failure.
23 February 1944:
Cdr Inaba Michimune (51) is appointed the CO of I-121 for the second time.
10 January 1945:
Lt Ueno Tadahiro (66) is appointed the Commanding Officer.
12 June 1945:
Transferred to the Maizuru Naval Base, Honshu in the Kure SubRonís SubDiv 33. Remains there until the end of the war.
15 August 1945:
Emperor Hirohito (Showa) broadcasts an Imperial Rescript that calls for an end to hostilities.
I-121 surrenders at Maizuru.
13 October 1945:
At Maizuru with I-201, I-202 and RO-500.
30 November 1945:
Removed from the Navy List.
30 April 1946:
Scuttled in Wakasa Bay.
- Bob Hackett and Sander Kingsepp.