This experiment investigated the effects of punishing shock-induced aggression in rats by increasing the intensity of shock when fighting occurred. Aggressor rats, given 1-mA ac inducing shocks, experienced 2-mA punishing shocks whenever they fought with a restrained target. Non-punished controls received the same frequency and duration of either 1-mA or 2-mA inducing shocks. During the first session, punished aggressors fought more frequently than 1-mA controls and also fought at a level which approximated that of the 2-mA controls. Only the control groups showed increases in fighting over the three sessions, and in the third session punished rats fought less than both control groups. This initial acceleration and delayed suppression of fighting by punishment was interpreted within the context of similar effects observed when punishment is superimposed on a variety of aversively motivated behaviors. The data suggest that punishment is an inefficient control procedure for aggression which is aversively motivated.