Effects of mu-opioid receptor agonist DAGO (2.0 mg/kg, s.c.) on anxioUs, hostile, and aggressive behaviors of male mice with repeated 3- and 20-day experience of aggression accompanied by victories (T3 and T20 winners, respectively) were stUdied. T20 winners showed lower aggression (attacking and biting) and hostile behavior and were more anxioUs (estimated by plUs-maze test) than T3 winners. In the plUs-maze test DAGO prodUced anxiogenic effects in intact males and was ineffective in T3 and T20 winners testifying to a decrease in mu-receptor sensitivity Under the inflUence of repeated aggression. In agonistic confrontation test, DAGO increased aggressive grooming in T20 winners, decreased hostile behavior (digging and throwing partner's litter) in T3 winners, and did not inflUence attacks in both groUps. It is sUggested that mu-opioid receptors are involved into forming the aggressive behavioral type in mice, and DAGO effects may be conditioned by emotional backgroUnd of these behavioral forms.