The neural mechanisms of predatory aggression in laboratory animals were investigated in a variety of rodents and members of the order Carnivora. Experimental enhancement of brain serotonin (5-HT) blocked killing behavior in rats, mice, mink and silver foxes, indicating that there is a 5-HT inhibiting mechanism of predatory aggression in animals of different species. Suppressed killing behavior, at least in some strains of mice, does not depend for expression on the inhibitory effect of the brain 5-HT system, but is caused by the low tonus of the system activating predatory behavior. Long-term satiation of mink increased the level of 5-hydroxyindole acetic acid in the lateral hypothalamus and amygdala and enhanced the latency of predatory aggression. It is suggested that 5-HT represents a dietary responsive endogenous factor regulating predatory behavior in carnivores. Selection of Norway rats over many generations for tamed behavior towards man (domestication) leads to an increase in level and turnover of 5-HT in the midbrain and hypothalamus, but does not change predatory aggression. Substantially reduced defensive behavior of domesticated rats is thus unconnected with the neural mechanism of predatory aggression.