We examined the effect of azimilide, a class III antiarrhythmic drug, on Na(+)/Ca(2+) exchange current (I(NCX)) in guinea-pig cardiac single ventricular cells. External application of azimilide suppressed bi-directional I(NCX) in a concentration-dependent manner. IC(50) values for outward and inward I(NCX) were 45 and 40 µM, respectively, with Hill coefficients of 1. Azimilide attenuated I(NCX) in the presence of trypsin in the patch pipette, indicating that azimilide is a trypsin-insensitive NCX inhibitor. Delayed afterdepolarization induced by electrical stimulation with ouabain disappeared in the presence of 30 µM azimilide. We conclude that azimilide inhibits NCX at supratherapeutic concentrations.