Some rats spontaneously and consistently kill mice, whereas others do not. In this experiment, mouse killing by rats was temporarily suppressed when punished by the administration of painful electric shock contingent upon killing, whereas shock uncorrelated with killing had little suppressive effect. Shock administered to suppressed killers in the presence of mice overcame the suppression produced by punishment and induced the rats to kill the mice. Thus shock both reduced the probability of killing on future occasions and induced killing. The same pattern of shock did not induce killing by rats that did not spontaneously kill mice prior to the experiment. (PsycINFO Database Record (c) 2006 APA, all rights reserved)