Bepridil, a novel active compound for prophylactic treatment of anginal attacks, induced persistent bradycardia and a non-specific anti-tachycardial effect, the mechanisms of which were investigated in vitro and in vivo. In vitro perfusion of bepridil in the life-support medium for isolated sino-atrial tissue from rabbit heart, caused a reduction in action potential (AP) spike frequency (recorded by KCl microelectrodes) starting at doses of 5 X 10(-6) M. This effect was dose-dependent up to concentrations of 5 X 10(-5) M, whereupon blockade of sinus activity set in. Bepridil at a dose of 5 X 10(-6) M, induced a concomitant reduction in AP amplitude (falling from 71 +/- 8 mV to 47 +/- 6 mV), maximum systolic depolarization velocity (phase 0) which fell from 1.85 +/- 0.35 V/s to 0.84 +/- 0.28 V/s, together with maximum diastolic depolarization velocity (phase 4) which fell from 38 +/- 3 mV/s to 24 +/- 5 mV/s. In vivo injection of bepridil at a dose of 5 mg/kg (i.v.) into 6 anaesthetized dogs which had undergone ablation of all the extrinsic cardiac afferent nerve supply, together with a bilateral medullo-adrenalectomy, caused a marked reduction in heart rate which fell from 98.7 +/- 4.2 beats/min to 76 +/- 5.3 beats/min sustained for more than 45 min. It is concluded that bepridil reduces heart rate by acting directly on the sinus node. This effect, which results in a flattening of the phase 0 and phase 4 slope, together with a longer AP duration, may be due to an increase in the time constants of slow inward ionic currents (already demonstrated elsewhere), but also to an increased time constant for deactivation of the outward potassium current (Ip).