The electrophysiological effects of intravenously administered bepridil, a new anti-anginal drug with slow calcium current blocking properties, were studied in 20 patients undergoing an endocavitary cardiac exploration for various reasons. Classical techniques of programmed electrical stimulation were used. Bepridil markedly increased the refractory period of the atrium, atrioventricular node and the AH- interval. The effects on HR and ventricular refractory period were minimal, and sinus rate decreased slightly. A pronounced reduction in the heart rate at which the Wenckebach phenomenon was obtained in the anterograde direction (16%) and particularly in the retrograde direction (26%) was observed. During this study the drug appeared beneficial in suppressing atrial flutter, atrial extrasystoles, and ventricular extrasystoles. In two patients with intranodal reciprocal rhythm, the injection of bepridil terminated the tachycardia which could not be reinitiated later. No side-effects occurred and it appears that bundle branch block and prolonged HV interval are not contraindications to the use of bepridil.