Systemically administered dopamine agonists have been shown to facilitate copulation in male rats. Microinjection of the dopamine agonist apomorphine into the medial preoptic area has also been reported to facilitate sexual behavior. The present experiments investigated the effects of medial preoptic microinjections of the dopamine antagonist cis-flupenthixol on male rat copulatory behavior. Fewer males initiated copulation and fewer ejaculated following flupenthixol administration. Those males that did ejaculate following flupenthixol injections had fewer ejaculations and longer interintromission intervals. Flupenthixol also antagonized the facilitative effects of apomorphine injections into the medial preoptic area. Flupenthixol and apomorphine produced only minor alterations in noncopulatory behaviors. The results suggest that dopamine receptors within the medial preoptic area are important in the regulation of masculine sexual behavior in the rat.