Effects of three treatments that induce mouse killing by rats were examined with cats. Food deprivation induced about 50% killing after 24 hr and almost 100% killing by 27 hr. Pilocarpine (at doses of 1.0 mg/kg that produced marked side-effects, and whether or not methyl atropine pretreatment blocked those side-effects) and chlordiazepoxide (at doses of 1.0--2.0 mg/kg) did not induce any killing. Pilocarpine produced a dose-related inhibition of spontaneous mouse killing (as it does in rats), but this was antagonised by methyl atropine. The side-effects of pilocarpine and chlordiazepoxide did not seem to account for their failure to induce killing. In contrast with food deprivation, the mechanisms by which pilocarpine and chloridazepoxide induce killing in rats may not have homologs in cats.