The objective of the study was to evaluate the acute effect of intravenous celiprolol on the electrophysiologic properties of the cardiac conduction system in man and to assess potential problems in terms of its causing heart block or sinus bradycardia. Eight patients with controlled coronary artery disease and hypertension but without conduction system disease were studied. All cardiac drugs e.g., digoxin, sympathomimetics and other beta blockers were discontinued prior to entering the study. Surface ECG leads I, AVF, and Vi were applied to each patient, and the ECG was continuously displayed on an oscilloscope. A quadripolar stimulating electrode (7F) was inserted percutaneously and positioned in the high right atrium and a tripolar or bipolar His bundle recording catheter was positioned cross the tricuspid valve. The atrial and His bundle electrograms were recorded on a direct writing recorder. After completion of the baseline electrophysiologic (EP) measurements, in an open fashion, celiprolol (0.1 mg/kg) was administered intravenously at the rate of 1 mg/min into each of the five patients. The complete battery of the EP measurements were repeated immediately following completion of the infusion. Preliminary data in eight patients demonstrated no significant effect of celiprolol on the cardiac conduction system.