The drug 3,4 methylenedioxymethamphetamine (MDMA; ecstasy) has a widely documented ability to increase feelings of love and closeness toward others. The present study investigated whether oxytocin, a neuropeptide involved in affiliative behavior, may play a role in this effect. A moderate (5 mg/kg, i.p.) dose of MDMA increased social interaction in male Wistar rats, primarily by increasing the amount of time rats spent lying adjacent to each other. MDMA (5 mg/kg) activated oxytocin-containing neurons in the supraoptic and paraventricular nuclei of the hypothalamus, as shown by Fos immunohistochemistry. MDMA (5 mg/kg i.p.) also increased plasma oxytocin levels and this effect was prevented by pre-treatment with the 5-HT(1A) antagonist N-[2-[4-(2-methyoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100,635; 1 mg/kg i.p.). The oxytocin receptor antagonist tocinoic acid (20 microg, i.c.v.) had no effect on social behavior when given alone but significantly attenuated the facilitation of social interaction produced by MDMA (5 mg/kg). The 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)-tetraline) (8-OH-DPAT, 0.25 mg/kg, i.p.) increased social behavior in a similar way to MDMA and this effect was also significantly attenuated by tocinoic acid. Taken together, these results suggest that oxytocin release, stimulated by MDMA through 5-HT(1A) receptors, may play a key role in the prosocial effects of MDMA and underlie some of the reinforcing effects of the drug.