The acute effects of (–)Δ9-trans-tetrahydrocannabinol (THC) were studied in several models of aggressive behavior. Chlorpromazine was twice as active as THC in blocking isolation-induced aggression in the mouse, and the ED50 values were 2 and 4 mg/ kg, respectively. Doses of THC which produced taming did not impair rotarod performance, but chlorpromazine caused motor depression at doses below those which attenuated fighting. Inhibition of foot shock-induced aggression in mice (ED50 = 22 mg/kg) and rats (ED50 = 14.9 mg/kg) by THC was considered to be related to the analgesic and /or motor depressant effects of the drug. There was an increase in the viciousness of septal rats 1 h after 20–80 mg/kg of THC; however, in another group of septal rats, THC (20 and 40 mg/kg) produced taming 3 h after administration. Imipramine (ED50 = 8.9 mg/kg) and THC (ED50 = 4.9 mg/kg) produced blockade of muricidal aggression in rats at doses below those impairing motor function. THC (5–20 mg/kg) did not block predatory aggression elicited by electrical stimulation of the hypothalamus in cats, but similar doses elevated the threshold current for stimulus-bound hissing (emotional response) beyond the control range in other cats.