Anabolic steroids and other androgens, such as testosterone propionate (TP), have a facilitatory role in the expression of aggressive behavior. Based upon literature indicating an inverse relationship between aggression and the central neurotransmitter serotonin (5-HT), the present study was undertaken to investigate the role of 5-HT in androgen-induced aggression. In this study, an animal model of aggression involving competition between male rat pairs for sugar pellets was used to investigate the effects of TP. When TP was administered daily (30 mg/kg) to nondominant rats, these animals became dominant. Dominant behavior was found to be stable throughout the study with continued daily administration of TP. To test the serotonergic component of TP-induced aggression, the serotonergic agonist 2-(1-piperazinyl) quinolone dimaleate (quipazine) was administered acutely to TP-dominant rats. Quipazine dose dependently reduced aggressive dominance in TP-dominant rats, as well as in naturally dominant rats. When the serotonergic antagonists pirenpirone or pizotyline were coadministered with quipazine to either group of dominant rats, they blocked the effect of quipazine in reducing dominance. However, when 1-[1H-Indol-4-yloxy]-3-[isopropylamino]-2-propanol (pindolol), a drug that acts at both beta-adrenergic receptors and at 5-HT1A and 5-HT1B receptors, was coadministered with quipazine there was a reversal of the quipazine effect on aggression only in TP-dominant rats. These results indicate that androgen-induced aggression may involve a complex alteration in serotonergic neurotransmission.