Intact and castrated male rats were kept for 1 week under constant or periodic light, during which time they received daily injections of either vehicle or testosterone propionate (125 or 250 micrograms day-1). Immediately after the experiment the rats were decapitated and serum and pituitary prolactin were measured radioimmunologically. The pituitary prolactin content was decreased after castration in both lighting conditions. Daily testosterone injections elevated the pituitary prolactin content of the castrated rats to the level of the intact rats in constant-light conditions but were ineffective under periodic light. Serum prolactin concentrations were not affected by constant light. We have previously shown that testosterone decreases the serum luteinizing hormone concentration of castrated rats more effectively under constant than under periodic light. In the present study we showed that serum prolactin was not elevated in constant light and thus could not be the cause of increased testosterone sensitivity. We also showed that testosterone elevates the pituitary prolactin content of castrated rats under constant but not under periodic light. This finding gives further support to the hypothesis that constant light sensitizes the hypothalamo-pituitary axis of castrated male rats to the effects of testosterone.