The effect of pancuronium bromide on atrioventricular conduction was studied by His-bundle electrocardiography during atrial pacing at different heart rates in halothane-anesthetized dogs. Pancuronium bromide, 0.1 mg/kg, uniformly enhanced atrioventricular conduction, as evidenced by decreased A-H interval (the time from the onset of the first high-frequency component of atrial depolarization to the His deflection). Wenckebach second-degree atrioventricular block appeared in five of 14 animals during atrial pacing with halothane anesthesia. This block was uniformly abolished by pancuronium. The enhancement of atrioventricular conduction may contribute to the development of tachyarrhythmias following administration of pancuronium in certain patients, particularly those who have atrial fibrillation or atrial flutter.