Mouse killing induced by septal lesions, olfactory bulb lesions, or parachlorophenylalanine (PCPA) injections was compared with that of sated or food-deprived spontaneous mouse-killing rats in order to evaluate whether the experimentally induced killing corresponds to killing that occurs spontaneously, which tends to be viewed as predatory. On the first mouse kill, the intensity of the initial reaction to the mouse, the site of the initial attack, and the time required to kill by all groups were similar except that bulbectomized rats required longer to kill. Following the kill, only rats with septal lesions and bulbectomized rats bit the mouse significantly more than spontaneous killers. With the second mouse kill, there was an increase in the intensity of the response to the mouse and a decrease in attack latency by all groups except the bulbectomized rats and the nondeprived spontaneous killers. When presented with a freshly killed mouse, rats with septal lesions attacked with the greatest intensity, but PCPA-injected rats and food-deprived spontaneous killers also attacked more intensely than nondeprived killers. When presented with a wad of cotton or a block of wood, there was little or no response from the animals of all groups. It is argued that the mouse killing induced by septal lesions or PCPA injections may be due to an enhanced predatory tendency similar to that occurring in food-deprived spontaneous killers. In contrast, the mouse killing by bulbectomized rats cannot be inferred to be predatory because their attacks were of low intensity and involved repeated superficial bites rather than one or two well-directed forceful bites.