Onychomys leucogaster (northern grasshopper mice) were induced to kill mice with response‐contingent shock, and the effects of several physiological, pharmacological, and endocrinological variables were assessed. Lesions of the septum facilitated mouse killing, while lesions of the amygdala abolished spontaneous mouse killing and delayed shock‐facilitated killing. Chlorpromazine (2.5–5 mg/kg) and chlordiazepoxide (5–10 mg/kg) facilitated mouse killing on postdrug trials but did not affect killing when the animals were drugged. Adrenalectomy, castration, and castration adrenalectomy did not alter the frequency of kill nor were sex‐related differences in killing noted. These results were compared to those found by others studying the effects of lesions and drugs on mouse killing by rats.