Maintaining adult male golden hamsters in short daily photoperiods (1hr of light and 23hr of darkness daily; LD 1.23) for 12 weeks caused the tests and accessory sex organs to atrophy and also led to significant depressions in pituitary LH and prolactin levels. If hamsters that were kept in LD 1.23 CYCLES RECEIVED WEEKLy subcutanceous implants of either a melanioninbeeswax or a 5-methoxtrypotophol-beeswax pellet (1 mgindole in 24 mg beeswax) the testes and accessory sex organs failed to involute and pituitary LH levels did not drop. Both treatments also retarded the depression in hypophyseal prolactin retarded the depression in hypophyseal prolactin levels. Treatment (twice daily on weekdays and once daily on weekends) with 1.5 mug injections of synthetic LRH (in 0.2 ml 8% gelatin) did not prevent gonadal or accessory organ atrophy while it further depressed hypophyseal LRH treatment, plasma LH titers were significantly elevated. The reproductive organs of hamsters that were moved from short (LD1;I 23) to long (LD 14;10) daily photoperiods regenerated within 8 weeks. This light-induced restoration of the gonads was not prevented or retarded by the weekly implantation of either melantonin-beeswax or 5-methoxytrptophol-beeswax pellets. The results suggest that in the male golden hamster neither melatonin nor 5-methoxtryptophol accounts for the antigonadotrophic activity of the pineal gland.