Cardiac electrophysiologic effects of a single oral dose of labetalol were determined in seven patients 4 to 9 days after a coronary artery bypass graft. Surface ECG and bipolar electrograms recorded from temporary pacing wires affixed to the normal right ventricle and abnormal left ventricle at the time of surgery were used to determine conduction intervals. Electrophysiologic parameters were recorded during fixed-rate atrial pacing. Sinus heart rate and blood pressure were monitored. Three patients received 100 mg and four patients received 200 mg labetalol. The drug had no significant effect on intraventricular conduction intervals or QRS duration. It did not significantly influence sinus heart rate or AV conduction time, but in two patients there was prolongation in AV conduction that may have been drug-induced. Labetalol induced a modest but significant decrease in systolic and diastolic blood pressure. In another study propranolol, unlike labetalol, had prolonged AV and intraventricular conduction in the abnormal left ventricle, but not in the normal right ventricle. The absence of these effects with labetalol may reflect lesser local anesthetic effect on intraventricular conduction and an alpha-adrenergic blocking effect that interferes with beta-blockade-induced prolongation of AV conduction.