Norepinephrine was given by continuous infusion at a rate of 80 microgram/kg/min for 2 min. Norepinephrine caused the appearance of cardiac arrhythmias, primarily ventricular extrasystoles. The prior administration of phentolamine, even in a dose which did not antagonize the pressor effect of norephinephrine, prevented extrasystole production by norepinephrine. Tolazoline and yohimbine, in doses which did not affect the vasopressor action of norepinephrine, also suppressed extrasystole production. It is concluded that the antiarrhythmic effect of phentolamine, does not require antagonism of alpha-adrenoceptors of the type found in the vasculature, but may involve effects on a different type of alpha-adrenoceptors in the heart.