The electrophysiologic effects of a toxic concentration of ouabain (10(-5) M) were studied in frog atrial trabeculae. The toxic concentration was determined by the appearance of a negative inotropic effect and an increase in basal tension. Current- and voltage-clamp measurements were performed. Ouabain did not alter the passive electrical properties of the preparation. Under current-clamp conditions the membrane depolarized and the action potential amplitude as well as its maximum rate of rise decreased. The current--voltage curve for the fast inward current was shifted toward more positive potentials and the maximum sodium current decreased. The maximum sodium conductance was also reduced. The process of reactivation of the fast inward current was accelerated. The slow inward current and the maximum slow conductance also decreased under ouabain. These effects could explain the negative inotropic action of high concentrations of glycosides, as well as the action potential changes observed by several investigators. They also help to understand the arrhythmogenic effects of high concentrations of digitalis.