Play behavior, as indicated by frequency and duration of pinning behavior, was studied in young rats between 18 and 64 days of age. The incidence of play was markedly increased by social isolation. Play increased from 18-28 days of age, peaked between 32 and 40 days of age, and gradually declined thereafter. Animals developed stable "dominance hierarchies" during the course of testing so that one animal pinned the other on the average 70% of the time. Also "dominant" animals exhibited the longer pin durations. The data indicate that social play can be efficiently studied in the laboratory rat and, further, that one function of play may be to establish stable social relationships.