The present series of experiments report the behavioral and endocrine responses of male and female rats to rat young. In the first experiment, the behavioral responses of intact male and ovariectomized female rats to foster young were measured. While both of these groups exhibited most components of parental behavior after four to six days of exposure to rat young, males failed to build good nests and to crouch over the young. In addition significantly more males killed young than did ovariectomized females. In the second experiment, treatment of male and virgin female rats with a 21-day hormone regimen of estradiol and progesterone stimulated an equally rapid onset of parental behavior in both sexes. Latencies to exhibit parental responses were approx. two days for both males and females. Pup killing was virtually eliminated in males by hormone treatment. Although the behavioral responses of hormone-primed males and females were similar, their prolactin responses to the presentation of rat young differed. Pup presentation stimulated an increase in plasma prolactin content in hormone-primed parental virgin females similar, but not identical, to that found in lactating control rats, while plasma prolactin levels in hormone-primed parental males did not rise in response to the presentation of young. These data indicate that a sex difference in neuroendocrine, but not behavioral, responsiveness to young exists between parental males and females.