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Feminized 2-year-old eels were reared in freshwater at 26℃ until October, then the water temperature was gradually decreased to 16℃ to December, and gradually increased to 26℃ from February to April. They received weekly injections of salmon pituitary extract (body weight, 15 or 30 mg/kg) to induce sexual maturation in September, December, or April. The quality of the eggs obtained in each season was evaluated by the resulting fertilization, hatching, and 8-day survival rates. High quality eggs were obtained via high-dose injections in most experimental groups. In the high-dose injection groups, eels were induced to the final maturation phase in all seasons; however, egg quality obtained in April was extremely low in contrast to that obtained in September and December. Histological observation of oocytes just before the process of artificial maturation was commenced indicated that the developmental stages of oocytes varied by season: oil droplet in September, early vitellogenic in December, and regressive from vitellogenesis in April. The extremely low egg quality in April is probably due to the fact that the oocytes had already entered the regressive stage. These results indicated that better quality of eel eggs can be obtained by starting the process of artificial maturation between September and December., 雌化ウナギ2 歳魚を9 月，12月または4 月にサケ脳下垂体注射によって人為催熟し，得られる卵質の差を受精率，孵化率および8 日後生存率で評価した。9 月，12月は比較的良質な卵が得られる傾向が認められたが，4 月では卵は得られるものの，その卵質は極めて悪かった。催熟前の卵母細胞を組織学的に観察すると，12月に卵黄形成開始が観察されが，4 月には退行している様子が観察された。4 月に催熟を開始しても良質な卵が得られない原因として，催熟前に既に卵母細胞が退行しつつあることが考えられた。以上の結果から，雌化ウナギ2 歳魚の人為催熟は9 月から12月の間にかけて開始することが良質な卵を得るために適当であると結論された。