The effects of the opioid-type stressor; immobilization, on severity of ounbain-induced cardiac arrhythmias and the possible involvement of serum catecholamines were investigated in rats. Immobilization significantly reduced the number of ventricular premature beats and the incidence of ventricular tachycardia episodes. The arterial serum catecholamine levels (A, NA and DA), measured immediately after the stressor application, were increased significicantly and were in negative correlation with the arrhythmia parameters. Both changes were reversed by naloxone (5 mg/kg) treatment after application of immobilization. The effects observed in this study may be attributed to the actions of endogenous opioid peptides released during stress.