Rat muricidal activity has been shown to be selectively inhibited by some antidepressants, antihistaminics and stimulants. Several agents from each class, including representative structural prototypes, were tested for anti-muricidal activity. Selective inhibition was considered only when it occurred at doses which were significantly lower than those impairing rotarod performance. The order of potency for agents found selectively active in this study was: tranylcypromine > methamphetamine > d-amphetamine > desipramine > imipramine > chlorpheniramine > pargyline. Structurally each of these drugs has a sidechain substitution which is an alkylamine. However, not all drugs with alkylamine sidechains were selectively effective. These studies seem to indicate those agents structurally related to d-amphetamine, i.e., methamphetamine and tranylcypromine, are the most potent selective inhibitors of mouse killing behavior in rats.