To examine the effects of the altered metabolic and hormonal state of diabetes mellitus on rodent social behavior, male Swiss Webster mice made diabetic with streptozotocin were tested in a resident-intruder encounter. Isolated diabetic and control mice were introduced as intruders into the home cages of aggression-trained resident mice. The encounters were videotaped and analyzed for frequencies and durations of agonistic behavior. Diabetic mice responded to the aggression of resident mice with significantly less investigation and aggression and significantly more static defense and escape behaviors than did control mice. Resident mice responded to less aggressive diabetic mice with more aggression and social investigation. Plasma corticosterone levels were significantly higher in diabetic mice compared to controls and positively correlated with submissive behavior in diabetic mice. These findings indicate that social behavior is altered in male diabetic mice and support the hypothesis that elevated pituitary-adrenal cortical activity and/or metabolic changes affect behavior in male diabetic mice.