The effects of surgical removal of the pineal organ on gonadal activity in the goldfish exposed to short or long photoperiod regimes were examined at different times of the year. From late winter to early summer, pinealectomy had a pronounced effect on gonadal activity. During this period, pinealectomy retarded gonadal activity in fish maintained on long photoperiods, but stimulated reproductive function in animals exposed to short photoperiods. Pinealectomy had no obvious effects on reproductive function from late summer to early winter. Our data suggest that the pineal organ of goldfish can be anti- or progonadal during certain seasons, depending on daylength conditions.