The present study was conducted to investigate the effects of adrenergic beta-blockers on agonistic behavior using quantitative ethological methods. In order to generate the agonistic behavior we employed the resident-intruder paradigm: An intruder male mouse is introduced into the home cage of a resident male mouse that has been cohabiting with a female for 5 weeks. The following drugs were administered orally to either resident or intruder mice: dl-Propranolol, oxprenolol, and carteolol. The injection-test interval was 60 min. Each test was recorded using a video TV monitor system, and at a later time several behavioral elements shown by both resident and intruder mice were measured. dl-Propranolol (5, 10, 20 mg/kg), oxprenolol (30, 50, 75 mg/kg), and carteolol (30, 50, 75 mg/kg) significantly suppressed the resident's aggressive episodes (offensive sideways posture, tail rattle, and attack bite) when resident mice were treated. By contrast, when intruder mice were treated with beta-blockers, aggressive episodes by untreated residents were not affected. The results suggest that dl-propranolol, oxprenolol, and carteolol have specific effects on the hostility of resident mice.