The whole-cell patch-clamp technique was used in adult mouse ventricular myocytes at 22 degrees C to study the transient outward current (I(to)) and its sensitivity to the antimycotics miconazole and clotrimazole, as well as to glybenclamide. I(to) elicited by depolarizing steps from a holding potential of -80 mV consisted of a fast inactivating component and a slowly inactivating component. In the presence of miconazole (IC50 of approximately 8 microM) or clotrimazole, I(to) peak amplitude was reduced and its inactivation accelerated, due to a selective suppression of the slow component, without an effect on the fast component or on the noninactivating current. The effect did not reverse upon washout, was not induced by intracellular drug application, and occurred without a change of the steady-state inactivation. In the presence of glybenclamide I(to) peak amplitude was reduced and its inactivation accelerated. In contrast to the antimycotics, glybenclamide suppressed both the fast and the slow components (IC50 of approximately 50 microM), its effect was reversible, and was associated with a negative shift of the steady-state inactivation. These data demonstrate a pharmacological separation of I(to) components using antimycotic drugs but not glybenclamide.