In order to determine the role of the pineal gland in mediating the effects of long-term light deprivation on prolactin (PRL) cell activity in a highly photosensitive species, PRL synthesis, storage and release were determined in both female and male Syrian hamsters that were either blind, blind and pinealectomized or left intact for 14 weeks. PRL release was determined in vivo by measuring the amount of immunoreactive (RIA) PRL in the serum of the animals in each group with a heterologous RIA for hamster PRL. Blinding resulted in a 98 and 88% reduction in serum PRL levels in female and male hamsters, respectively. Pinealectomy largely prevented the suppressive effects of blinding on PRL release; however, PRL levels in blind pinealectomized animals were intermediate between those in intact and blind animals. PRL synthesis was evaluated by assessing the amount of ³H-leucine incorporated into PRL by anterior pituitaries in vitro. In female hamsters 14 weeks of light deprivation resulted in an 87% decrease in the incorporation of ³H-leucine into newly synthesized PRL whereas in males only a 40% reduction occurred. While pinealectomy completely prevented the inhibitory effects of blinding on PRL synthesis in males, it was less effective in female hamsters inasmuch as PRL synthesis was still nearly 50% lower in blind pinealectomized animals than in controls. Stored PRL, as represented by the total amount of RIA-PRL in vitro, was 96% lower in blind female hamsters as compared with intact controls; in blind male hamsters, stored PRL was reduced by 77%. As in the case of PRL synthesis, pinealectomy completely prevented the suppressive effects of blinding on PRL storage in males; however, it was only partially effective in female animals since total RIA-PRL was still 58% less than in controls. In conclusion, it appears that the inhibitory effects of light deprivation on PRL cell activity are more severe in female than in male hamsters. Furthermore, while the suppressive effects of blinding on PRL cell function in males seem to be pineal-dependent, they appear to be at least partially pineal-independent in female animals.