The l- and d-isomers of verapamil were selectively perfused into the sinus node artery and atrioventricular (AV) node artery of 48 dogs. Injection of l-verapamil into the sinus node artery during sinus rhythm and into the AV node artery during AV junctional rhythm depresses both sinus rhythm and AV junctional rhythm significantly more than does the d-isomer. l-Verapamil is three to four times more powerful than d-verapamil. Injection of the isomers into the AV node artery during sinus rhythm rapidly impairs AV conduction. Increments in conduction time are measured exclusively at the level of the A-H interval of the His bundle electrogram, and l-verapamil is six times more powerful than d-verapamil. Neither d- nor l-verapamil in concentrations that exert a profound negative chronotropic effect or cause AV block, has any significant effect on transatrial or His bundle conduction. Thus these concentrations of d-verapamil have little or no significant effect on the fast sodium channel, but both verapamil isomers affect the slow channel. The main difference in action between l- and d-verapamil appears to be only quantitative in nature. The sinus node is significantly more sensitive to the negative chronotropic action of verapamil than is the AV junctional pacemaker, and this differential responsiveness appears to be related to the different intrinsic rates of the two pacemakers. During sinus rhythm (either in the presence or absence of atropine) sinus node automaticity is less affected than AV conduction when verapamil is given parenterally. We propose that this greater negative dromotropic effect of verapamil is also in part due to a rate-dependent process, since during sinus rhythm AV junctional cells have to be depolarized at frequencies significantly higher than their intrinsic rates.