Matched litter mates were reared in one of three conditions: in pairs or in isolation with or without one hour of daily playfighting experience from 20 to 50 days of age. The rats were then regrouped within condition so that they lived with identically reared cagemates for a month. This regrouping eliminated the transient effects of isolation such as increased fearfulness. When tested as adults, there was no effect of early rearing condition on the probability of intraspecific aggression or muricide, although isolation‐reared rats were less likely to retrieve the mice. However, isolation rearing reduced the latency to initiate shock‐induced defensive aggression and increased both its frequency and intensity. Isolated animals which had been given daily playfighting during development did not show the effects of early social deprivation. The mechanisms through which playfighting experience shapes later defensive behavior remain to be determined.