The general objective of this research has been to enhance our understanding of the influence of the autonomic nervous system on electrophysiologic properties of the heart. We have focused our attention on two areas of the heart; namely, Purkinje fibers and the sinoatrial node. Studies have been carried out both in intact heart as well as in isolated tissues removed from the heart and studied with the microelectrode technique. We have demonstrated that at low external potassium concentrations, propranolol has a hyperpolarizing effect on Purkinje fibers, that this effect is independent of its Beta-blocking action and that it decreases the membrane input resistance. These data suggest that propranolol increases the potassium conductance during diastole. These studies are designed to provide an explanation for the mechanism of action of propranolol on Purkinje fibers and the basis for understanding its antiarrhythmic influence. A second group of studies have been directed at the analysis of sinus node response to premature stimulation. Comparable techniques have been employed in the intact heart and in the isolated rabbit right atrial preparation. These studies have provided a basis for the estimation of the sinoatral conduction time, determining the presence of sinus node echoes, furthering our understanding of the effects of acetylcholine on different atrial cell types. In addition, our studies have provided further data on the factors that regulate maximum diastolic potential in cardiac Purkinje fibers.