In the anesthetized dog, the effects of ouabain injected intravenously and directly into the atrioventricular (A-V) node artery were studied. Automaticity within the A-V junction was decreased after a dose of ouabain, 30 μg/kg given intravenously (i.v.), whereas automaticity of ventricular specialized fibers was increased after a dose of 40 μg/kg i.v. The decrease of automaticity within the A-V junction was accompanied by an increased duration of the functional refractory period of this structure and by a prolongation of conduction time through the A-V junction as determined from the His bundle electrogram.