The effect of pinealectomy on reproductive function was examined in the Turkish hamster, Mesocricetus brandti. Pinealectomy resulted in testicular regression in this species. This result was unexpected since pinealectomy prevents short day-induced regression of the gonads in the closely related Syrian hamster, Mesocricetus auratus. Decentralization of the superior cervical ganglia and exposure to continuous illumination also caused testicular regression in the Turkish hamster. These manipulations are believed to block pineal melatonin synthesis. In each case (i.e. pinealectomy, decentralization of the superior cervical ganglia, exposure to continuous illumination), the testes regressed after approximately 3--9 weeks and underwent recrudescence after approximately 16--28 weeks. This cycle of testicular regression and recrudescence was similar to that observed in Turkish hamsters exposed to a short day photoperiod. In further experiments, the effects of exogenous melatonin were studied in Turkish and Syrian hamsters. The results of these studies suggest that, in Turkish hamsters, pineal melatonin may be involved in both the maintenance of testis function during exposure to a long day photoperiod and also in the suppression of reproductive function in short days. This is in contrast to the Syrian hamster, in which melatonin appears to be important only for inhibition of gonadal function in short days.