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Gonadal maturation, ovulation and spawning in the Japanese killifish, Oryzias latipes, are especially influenced by environmental changes of photoperiod (Egami, 1954; Yoshioka, 1962). Reduction in gonosomatic index (GSI: gonadal weight as a percentage of total body weight) is marked in fish maintained under continuous darkness or natural daylight (a short photoperiod) conditions during the period from October to December, but the index increases after pinealectomy (Urasaki, 1972). In Oryzias it is obvious that the reduced light period is effective in lowering gonadal weight, but the mechanism is still unknown. However, it is postulated that the pineal gland plays some role in the mechanism. The present investigation was carried out to study the mode of action of the pineal in inhibiting gonadal development in the fish.

Adult specimens, 360–450 mg in body weight, of Oryzias latipes were used in the experiment. On 12 November (Expt 1) and on 20 January (Expt