The electrophysiological effects of acebutolol were studied in 12 patients. Heart rate and systemic arterial pressure were monitored. Plasma acebutolol levels were estimated. The first 6 patients (group A) received 0.3 mg/kg body weight I.V. The subsequent 6 patients (group B) received 0.5 mg/kg body weight I.V. Acebutolol delayed both antegrade and retrograde conduction through the AV node. The effect was greater in group B patients. Effective and functional refractory periods of the AV node were also increased after acebutolol. Intraatrial and intraventricular conduction and the effective refractory periods of the atrium and ventricle were not affected. Acebutolol did not alter significantly the sinus node recovery time or the QTc interval. Heart rate fell, especially where initial rates were fast. Systolic arterial pressure was lowered in group B patients. Peak mean levels of acebutolol were reached 5 min after injection of the drug. In the doses used, acebutolol, although well tolerated, failed to demonstrate electrophysiological evidence of a membrane-stabilizing effect and is therefore unlikely to add to the areas of success achieved by other beta-blocking drugs in the management of arrhythmias.