We examined the effect of azimilide, a class III antiarrhythmic drug, on Na⁺/Ca²⁺ 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. IC₅₀ 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.