Three experiments were conducted to assess the role of endogenous opioids in controlling mating behavior and sexual reward in the male rat. In Experiment 1 SC administration of naloxone (0.5, 1.0, 5.0, or 10.0 mg/kg) significantly reduced mounting and ejaculation in male rats tested 14, but not 7 days, after castration. In Experiment 2 naloxone (5.0 mg/kg) administered SC to gonadally intact males, which had ejaculated repeatedly with one female until they were sexually sated, significantly inhibited the resumption of mating after the reintroduction of a female partner. One interpretation of these results is that naloxone attenuated the reward experienced by castrated and sexually sated males in the presence of an estrous female, thereby disrupting males' coital performance. This hypothesis was tested in Experiment 3 using a conditioned place preference paradigm in which males copulated with an estrous female in an initially "non-preferred" (white) compartment, whereas on alternate days they remained alone in an initially "preferred" (black) compartment. After 10 such conditioning sessions, males were either castrated or sham-operated. They later were given free access to both compartments in the absence of an estrous female. Seven days after conditioning and surgery, sham-operated, naloxone-injected males and both groups of castrates spent significantly less time than sham-operated, saline-injected controls in the initially "non-preferred" compartment. Fourteen days after conditioning and surgery castrated, naloxone-treated males spent significantly less time in the "non-preferred" compartment than males in the other three groups. Endogenous opioids may play an important role in the interpretation by males of the incentive motivational stimuli which emanate from an estrous female.