Effects of bepridil, a new antianginal and potential antiarrhythmic agent, on transmembrane action potentials of ventricular muscles were examined in isolated right ventricular papillary muscles of guinea-pig. Bepridil at concentrations above 5 X 10(-6)M caused a dose-dependent decrease in both the maximum upstroke velocity (Vmax) and the action potential duration from the upstroke to 30% repolarization ( APD30 ). On the other hand, the resting potential (RP), the amplitude of action potential (AMP), and the action potential duration from the upstroke to 90% repolarization ( APD90 ) were not affected even at the highest concentration applied (10(-5)M). The curves relating membrane potential and Vmax were shifted by bepridil at 5 X 10(-6)M along the voltage axis in the direction of more negative potentials. The recovery kinetics of Vmax assessed by premature stimuli were definitely slowed by bepridil at above 10(-6)M. This effect was more pronounced with higher [K+]o (10 mM) than normal [K+]o (5 mM). Bepridil at 5 X 10(-6)M caused a rate-dependent decrease of Vmax (use-dependent block) with rapid onset and offset, as did lidocaine. Slow responses, which had been induced by isoprenaline (5 X 10(-6)M) in K+-depolarized preparations, were suppressed significantly by additional application of bepridil at 10(-5)M. These findings suggest that bepridil has electrophysiological characteristics similar to those both of Class Ib and Class IV antiarrhythmic drugs.