Researches on play-fighting in rats, including issues on its behavioral characteristics, developmental change, and the functions of it were reviewed. It is suggested that the sequential analysis of behavior elements that construct play-fighitng is helpful in discriminating play-fighting from the serious fighting, though other investigations for characterizing this behavior pattern are also required. A recent study on ultrasonic vocalization which is emitted during play-fighting behavior is also considered as a way of characterizing play-fighting. More precise analyses on the relaionship between such vocalizations and agonistic behavior may help to identify the play-fighting behavior with higher accuracy. Studies on the developmental change of play-fighting indicates that it has little function of practice for the aggressive behavior in later adult stage, as has been supposed. Rather it might contribute to the normal development of the later sexual behavior, since some behavior elements shown by male juveniles appear to reflect the preparative stage of the reproductive behavior.