Male golden hamsters were exposed to long photoperiod or short photoperiod (SP) and injected with 1 µg TRH and/or 1 µg LHRH at lights on (LO) or lights off (LX) for a total of 8 weeks. Both TRH and LHRH prevented testicular regression if they were injected at LO. Injected at LX, TRH did not prevent testicular regression, and LHRH was only partially effective. Plasma β-endorphin levels were significantly higher in groups with atrophic testes. These results indicate that TRH like LHRH can prevent SP-induced testicular regression in hamsters by some unknown mechanism and that β-endorphin may be involved in the control of testicular function in hamsters.