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In adult male hamsters, 2 months of exposure to a short photoperiod (5 h of light:19 h of darkness) caused testicular regression and a precipitous decline in plasma PRL, in agreement with earlier reports from other laboratories. Depressed release of PRL cannot be explained by a reduction in testicular steroidogenesis, because castration of males kept in a long photoperiod did not reduce PRL levels and administration of testosterone to males kept in a short photoperiod failed to reverse the decline in plasma PRL concentration. Treatment of such "regressed" animals with PRL, GH, or ectopic pituitary transplants stimulated growth of the testes and the accessory reproductive glands, increased the concentration of LH receptors in the testes, and elevated plasma testosterone levels. A single injection of 250 microgram PRL was sufficient to increase testicular LH binding, and chronic treatment with pituitary grafts completely reversed testicular regression. The effectiveness of exogenous PRL in stimulating testicular growth and LH receptors was significantly influenced by the timing of the injection. In some experiments, gonadotropin levels appeared elevated in animals injected with PRL, but these differences were not statistically significant. In hamsters with gonadal regression induced by exposure to a short photoperiod, daily administration of 20 microgram H and/or 150 microgram FSH had no apparent effect on testicular function. However, treatment with large doses of hCG and/or PMS gonadotropin resulted in significant stimulation of testicular growth and steroidogenesis. Chronic treatment of males maintained in a long photoperiod (14 h of light:10 h of darkness) with an inhibitor of PRL release, 2-Br-alpha-ergocryptine, resulted in a decreased weight of the testes and seminal vesicles. Administration of this inhibitor for a longer period (2 months) produced a significant increase in body weight but had little effect on testicular function. These results indicate that changes in the release of PRL (and possibly also GH) may plan an important role in mediating the effects of the photoperiod on testicular function in the golden hamster.