Research is reported which investigated the aggressive interactions between adult male rats in the presence of 17-27 day old juvenile conspecifics. Following a brief exposure to an inaccessible juvenile male, juvenile female, estrous adult female, or empty compartment, the males of Experiment 1 were allowed to interact with another adult male. Results were that the presence of both genders of juveniles reduced fighting between the adult males, and pre-pubertal females were particularly effective as an aggression-inhibitor. The hormonal bases for those sex differences were examined in Experiment 2 using juveniles gonadectomized on Postnatal Day 10. Adult males were more aggressive following exposure to intact juvenile males than to castrated juvenile males; the latter males, intact juvenile females, and ovariectomized juvenile females provoked similar and low rates of fighting between adult males. These data suggest that the sexual dimorphism is a function of testicular hormones present during the juvenile stage which modify the stimulus characteristics of a pre-pubertal male rat.