The acute effects of intravenous nadolol (0.01 and 0.02 mg/kg) on cardiac electrophysiologic parameters were assessed with His bundle recording and programmed atrial stimulation. The higher dose of nadolol reduced resting heart rate (71 vs. 65 beats/min, P less than 0.02), and the degree of slowing was related to the initial heart rate (r = -0.68, P less than 0.05). Atrioventricular conduction time as defined by the paced A-H interval, rose by 12 msec (P less than 0.001) after nadolol (0.02 mg/kg) administration. Atrial refractoriness increased (by 10 msec, P less than 0.02) only at the higher dose level with nadolol. At both dose levels, atrioventricular nodal effective and functional refractory periods were increased (P less than 0.02) by a mean of 45 and 21 msec, respectively, suggesting greater sensitivity of atrioventricular nodal refractoriness to beta-adrenergic blockade. Nadolol's effects were generally similar to those of previously reported studies with other beta-adrenergic blockers. These data suggest that nadolol slows conduction through the atrioventricular node and increases atrial and atrioventricular nodal refractoriness.