Chlordiazepoxide HCl, at dose levels from 2.5 mg/kg to 80 mg/kg, significantly increased the low base rates of mouse killing (3-9%) observed in large samples (N = 100/dose) of Holtzman strain albino male rats. Maximal killing rates were obtained at doses from 7.5 mg/kg to 20 mg/kg. Diazepam was equally effective, and several times more potent than chlordiazepoxide. Pentobarbital did not increase killing. Killing induced by chlordiazepoxide was blocked by d-amphetamine SO4, but not by l-amphetamine, at dose levels similar to those that block undrugged killing in this strain (ED50 = 1.5 mg/kg). Unlike pilocarpine-induced killing, the effects of chlordiazepoxide were not increased or decreased significantly by either peripherally or centrally active anticholinergic drugs, over wide dose ranges of these agents; nor were the effects of chlordiazepoxide increased by repeated daily administration.