Male sexual behavior results in induction of Fos-immunoreactivity (IR) in neurons in the medial preoptic nucleus (MPN) of the male rat. In addition, a recent finding from our laboratory demonstrated mating-induced activation of mu-opioid receptor (MOR) containing neurons, as indicated by internalization of this receptor (Coolen et al, SFN abstr. 420.4, 1999). This suggests that release and binding of endogenous opioid peptides (EOP) in MPN occurs as a result of male sexual behavior. The present study addressed two questions: (1) does mating induce Fos-IR in MOR-containing neurons, and (2) is Fos-induction a result of EOP binding to MORs? Male rats were injected s.c. with saline or the opioid receptor antagonist naloxone (5 or 10 mg/kg bodyweight), 30 minutes before mating which included one ejaculation. One hour after mating animals were sacrificed and brains immunostained for c-fos and MOR. Indeed, following mating approximately 50% of MOR containing neurons in MPN were Fos positive. However, naloxone did...