The electrophysiologic effects of dextro (d)- and levo (l)-verapamil on sinoatrial (SA) and atrioventricular (AV) node function were studied in ten patients undergoing electrophysiologic evaluation of their supraventricular tachyarrhythmias. Both isomers elicited a significant prolongation of sinus node recovery time (SNRT) and AH interval. No difference between d- and l-verapamil regarding the magnitude of the effects were observed. However, 50 mg d-verapamil was required to elicit the same electrophysiologic effects as 5 mg l-verapamil. The d- and l-verapamil plasma concentrations associated with the maximum effect on AH interval and SNRT showed a more than 20-fold difference (d: 380 ng/ml; l: 19 ng/ml). These data demonstrate that both verapamil isomers possess qualitatively similar slow channel blocking effects on the SA and AV node in humans, but the l isomer is 20 times more potent than the d isomer.