The effects of mating on plasma levels of oxytocin and prolactin in male and female rabbits have been investigated. Blood was collected through indwelling cardiac catheters at intervals of 1, 2, 4, 6, 12, 24, 36 and 60 min after mating. In female rabbits additional samples were taken 5 h after mating, as well as daily during the ensuing pregnancy or pseudopregnancy. They were also fitted with intrauterine balloons for recording uterine activity. Copulation induced a rapid, transient rise in plasma oxytocin in female rabbits at the same time as a fall in plasma prolactin. Mating or sexual excitement had no significant effect on plasma concentrations of oxytocin or prolactin in bucks. Relatively large fluctuations of plasma oxytocin were seen in male rabbits under normal conditions and after mating, suggesting episodic release of oxytocin in a random fashion. The uterine recordings indicated that, in spite of the modest release of oxytocin, a strong sympathetic adrenal activation occurred in response to mating and this provided the overriding influence on uterine activity. During pregnancy plasma levels of prolactin rose significantly on day 4, and remained raised throughout most of gestation. Plasma prolactin fluctuated widely during the first half of pregnancy but the mean levels were higher than those found during the second half of gestation. When pseudopregnancy was induced with injection of an ovulating dose of LH, plasma prolactin rose in a similar manner as during early gestation or mating-induced pseudopregnancy. Thus, in contrast to rats, stimuli associated with mating have no direct influence on the subsequent release of prolactin in rabbits. The secretion of prolactin during gestation seems to be controlled entirely by ovarian steroids, probably progesterone.