The seasonal changes in the activity of the melatonin synthsis were studied with the cultivated pineal body of Zacco temmincki. The activity of melatonin synthesis was measured by the radioactivity of the labeled melatonin secreted into culture medium using 14C-5-hydroxy-tryptamine as a precursor.
Gonad somatic index of the female fish showed a big value from June through August and that of the male became maximum in June and had become small after that. The fishes that used in this experiment showed the delaied spawning season for one month than ordinary year.
The synthetic activity of melatonin in Zacco temmincki pineal body synchronized with light−dark cycle（LD12:12）with low levels in the light period and high in the dark one. The difference of the activity between the light and dark was not seen from October through March. Seasonal change was detected in the synthetic activity of melatonin low in winter and high in summer. The rhythms of the melatonin synthetic activity in constant darkness were seen in only August and September, but not in other months through the year. These results indicate clearly that the circadian rhythm exists in the melatonin synthesis of the pineal body in Zacco temmincki.