Out of 12 patients in whom phenylephrine terminated ventricular tachycardia, four were selected for detailed studies of its mechanism of action. Pretreatment with edrophonium (15-20 mg, i.v.) decreased, while atropine (2.4 mg, i.v.) increased by at least a factor of two, the dose of phenylephrine required to break ventricular tachycardia. Carotid sinus massage following pretreatment with edrophonium in unusually high (15-20 mg, i.v.) doses broke ventricular tachycardia in all four patients. The evidence presented supports the assumption that a vagal mechanism caused both instances of termination. These findings significantly alter our interpretation of vagal interventions in the bedside clinical diagnosis of wide QRS complex tachycardias.