Abstract. Social interaction between juvenile male rats that were unfamiliar with one another (JNI) was compared with that of adult unfamiliar animals (ANI). In their home cages, subjects were confronted with a stimulus conspecific and their agonistic behavior patterns were analyzed. It was found that behaviors that characterize "play fighting" (pouncing, submissive posture, on-top posture, etc.) were observed more in JNI than in ANI, whereas behaviors found in serious fighting (pawing, upright posture, etc.) were shown more in ANI. Analyses of the transitions between behaviors also revealed that the transitions constituting play fighting were found only in JNI. On the other hand, transitions that characterize serious fighting were shown in both JNI and ANI. These findings are discussed along with those of a previous study that observed the agonistic behavior between littermates, and the existence of some cue that signals the play-fighting situation among rats is suggested.