Induction of the c-fos protein product (Fos) was used to immunocytochemically identify oxytocin (OT) neurons that may be activated during copulatory interactions. Fos induction was quantified in sexually-experienced male rats after either (a) exposure to a testing arena recently vacated by an estrous female, (b) copulatory interactions such as mounting and intromission without ejaculation, or (c) mounting and intromissions culminating in ejaculation. In the parvocellular regions of the paraventricular nucleus of the hypothalamus (PVN), the number of neurons expressing Fos increased following either intromission (53%) or ejaculation (124%). Significant, but less striking, increases in the number of cells expressing Fos were noted in magnocellular regions of the PVN where intromission resulted in a 13% increase and ejaculation in a 49% increase in Fos. The number of perikarya immunoreactive for OT and AVP did not differ as a function of increasing sexual contacts. In control (novel arena) males, 33-73% of the Fos labeling occurred in OT cells. Sexual interactions did not enhance the number of double-labeled cells in most parvocellular regions. However, in lateral parvocellular regions located in the most caudal aspects of the PVN, 31% of the Fos-positive cells occurred in OT neurons in ejaculated males, while in control males none of the OT cells were double-labeled. This PVN subdivision is known to consist of neurons that project to the brain stem and spinal cord at lumbar levels which contain motor neurons that regulate penile reflexes. The present data suggest a possible neurochemical circuit which incorporates oxytocinergic neurons in the mediation of masculine sexual responses.