Atria attached with functional sympathetic postganglionic fibers were isolated from rabbits and placed in modified Ringer's solution at 30°C. Transmembrane potentials were recorded from pacemaker fibers of the S-A node. Typical effects of sympathetic nerve stimulation were an increase in the slope of slow depolarization during diastole and an acceleration of the pacemaker rate. In about one-fourth of the pacemaker fibers studied, nerve stimulation did not induce the increase in the slope in spite of marked acceleration of pacemaker rate. The 90% duration was decreased and the depolarization time was prolonged after the stimulation. Similar changes in membrane potentials were induced by the application of norepinephrine. However, the incidence of pacemaker shift was observed more in the presence of norepinephrine than in response to sympathetic stimulation. When the threshold potential was lowered because of the conversion of the true pacemaker fiber to a latent pacemaker fiber, the size of the overshoot was not increased. Similar changes in the action-potential duration and the depolarization time in response to sympathetic stimulation and norepinephrine application were elicited when the S-A node was driven at interstimulus intervals of less than 370 msec. Cardiac norepinephrine might not induce changes in membrane potential during systole but during diastole.