Left or right stellate ganglion stimulation resulted in changes of ST segments and T waves of electrocardiogram. The present experiments were performed in an attempt to elucidate α- and β-adrenergic actions, and calcium action on the monophasic action potential duration (APD), that is, the repolarizing phase of myocardium. During experiments in open-chest dogs, right ventricular pacing was performed for avoiding the acceleration of heart rate by stellate ganglion stimulation. Administration of phentolamine, propranolol or D600 prolonged APD, while hydrazine shortened it. However, stellate ganglion stimulation caused a prolongation of APD after infusion of propranolol or hydrazine, and a shortening of APD after infusion of phentolamine, D600 or hydrazine. These results indicate that α- and β-adrenergic actions as well as calcium ions can be related to APD in the ventricular myocardium of dogs : apparently, α-adrenergic action prolongs APD and, β-adrenergic action shortens it.