Adult male rats were treated chronically with haloperidol (1 mg/kg) daily or propranolol (10 mg/kg bid) and evaluated for changes in shock-induced fighting. Haloperidol suppressed fighting. Chronic propranolol facilitated fighting when rats were tested eight hours after injection. Acutely, either 5 or 10 mg/kg of d,1-propranolol suppressed shock-induced fighting. Chronic pindolol (10 mg/kg bid) and chronic 1-propranolol (5 mg/kg bid) administration increased fighting. Chronic d,1-propranolol, 1-propranolol or pindolol administration was associated with an increase in Bmax for beta-adrenergic receptors. No change in fighting or Bmax was observed with the chronic administration of d-propranolol (5 mg/kg bid) or metoprolol (10 mg/kg bid). This increase in shock-induced fighting appears to be a behavioral response developing as a consequence of increased beta-adrenergic receptors responding to endogenously released norepinephrine.