Whereas prolactin (PRL) release increases in female rats as a result of stimuli from rat young, parental male rats fail to show a rise in plasma PRL levels in response to pup stimuli. In the present study, we have characterized two aspects of the endocrine control of PRL release in male and female rats in an attempt to account for this sex difference in pup-induced PRL secretion. First, we compared the patterns of PRL secretion throughout a prolonged steroid priming treatment that effectively stimulates the onset of parental responsiveness in both sexes. Second, we measured pituitary responsiveness to a PRL releasing agent (thyrotropin releasing hormone: TRH) in male and female rats that had previously experienced the steroid treatment. We found that in the presence of estradiol alone or soon after the onset of progesterone treatment in the presence of estrogen, female but not male rats displayed afternoon elevations in PRL. We also found that, following steroid treatment, TRH caused significant and comparable elevations in plasma PRL in both sexes. These experiments demonstrate differences in patterns of PRL secretion in male and female rats during the early phase of a steroid priming treatment that induces parental responsiveness, yet comparable pituitary sensitivity to TRH-induced PRL release following this 3 week steroid treatment. These data suggest that sex differences in pup-induced PRL secretion in rats do not result from fundamental differences in endocrine priming or from sex differences in pituitary responsiveness.