The dose-response effects of ketamine, an intravenous anesthetic with psychotomimetic properties which is a ketone derivative of phencyclidine, were evaluated in several experimental animal models of aggression. Low doses of ketamine (3 and 5 mg/kg) administered intraperitoneally showed a tendency to increase aggressiveness in REM sleep-deprived, food-deprived + apomorphine (20 mg/kg) pretreated rats, and in socially isolated mice. Ketamine (10 mg/kg) blocked aggressive behavior in food-deprived, apomorphine-pretreated rats probably because of locomotor activity impairment. Ketamine (3, 5 or 10 mg/kg) did not induce mouse-killing behavior in non-muricide rats. However, ketamine inhibited predatory aggression of muricide rats at a dose which did not affect ambulation (3 mg/kg). These data show that ketamine, like phencyclidine, has different effects on aggressive behavior which depend on the experimental model used.