To elucidate the relative contribution of somatodendritic 5-HT(1A) autoreceptors and postsynaptic 5-HT(1A) receptors in the specific anti-aggressive properties of 5-HT(1A) receptor agonists, the influence of the novel benzodioxopiperazine compound S-15535, which behaves in vivo as a competitive antagonist at postsynaptic 5-HT(1A) receptors and as an agonist at 5-HT(1A) autoreceptors, upon offensive and defensive aggression was investigated in wild-type rats using a resident-intruder paradigm. S-15535 exerted a potent dose-dependent decrease in offensive, but not defensive, aggressive behavior (inhibitory dose (ID)(50) = 1.11 mg/kg). This anti-aggressive profile was roughly similar to that of the potent pre- and postsynaptic 5-HT(1A) full agonist alnespirone (ID(50) = 1. 24). The drug's profound anti-aggressive actions were not accompanied by sedative side effects or signs of the "5-HT(1A) receptor-mediated behavioral syndrome," which are characteristically induced by prototypical 5-HT(1A) receptor agonists like 8-OH-DPAT and buspirone. The selective pre- and postsynaptic 5-HT(1A) antagonist WAY-100635, which was inactive given alone, abolished the anti-aggressive effects of S-15535 and alnespirone, thereby confirming the involvement of 5-HT(1A) receptors. Furthermore, combined administration of S-15535 and alnespirone elicited an additive anti-aggressive effect, providing further support for somatodendritic 5-HT(1A) receptor involvement. Finally, the postsynaptic 5-HT(1A) antagonistic properties of S-15535 were confirmed by showing blockade of the alnespirone-induced hypothermia, a postsynaptic 5-HT(1A) mediated response in the rat. These data provide extensive evidence that the anti-aggressive effects of 5-HT(1A) receptor agonists are expressed via their action on somatodendritic 5-HT(1A) autoreceptors, thereby most likely attenuating intruder-activated serotonergic neurotransmission.