Experiments with social instigation or the omission of scheduled reinforcement show that serotonergic mechanisms may be involved in escalated aggression in animals. 5-HT1B receptor agonists have anti-aggressive effects in individuals who show moderate as well as high levels of aggression. The present study compared the effects of the 5-HT1B agonist anpirtoline (0.125-1.5 mg/kg) on (1) species-typical aggressive behavior in male mice, (2) aggression "instigated" or primed by prior exposure to the opponent, and (3) aggression heightened by "frustration" caused by omission of scheduled reinforcement. The effects of anpirtoline on species-typical behavior were also assessed after pretreatment with the 5-HT1B/1D receptor antagonist GR127935 (10 mg/kg). Anpirtoline, like other 5-HT1B agonists (CP-94,253, zolmitriptan), decreased both instigated and frustration-heightened aggression, while motor behavior was unaffected. The aggression-inhibiting effects of anpirtoline were blocked by pretreatment with GR127935. The current results indicate that the 5-HT(1B) receptor is critically involved in the modulation of escalated aggression.