In Experiment 1, the 5-HT uptake blocker fluoxetine (FLX; 20 mg/kg) reduced the proportion of sexually experienced male rats displaying ejaculations. Among those animals that did ejaculate there was an increase in intromission frequency (IF), ejaculation latency (EL), and postejaculatory interval (PEI) and a reduction in copulatory efficiency (CE) during the final copulatory sequence prior to sexual exhaustion. In Experiment 2, we found similar inhibitory effects of FLX as well as facilitating effects of lesions of the nucleus paragigantocellularis (PGi) on male rat copulatory behavior. Males with PGi lesions displayed more ejaculations and a longer latency to sexual exhaustion compared to intact animals. When FLX was given to rats with PGi lesions, it did not influence the proportion of rats ejaculating nor did it alter IF, EL, or PEI during the final copulatory series prior to exhaustion. These findings suggest that the inhibitory influences of FLX on male rat copulatory behavior are mediated in part by the interaction of FLX with neurons originating in the PGi.