The electrophysiologic effects of ORG 30701 (1-8 x 10(-6) M) were tested in a thin, two-dimensional sheet of ventricular myocardium. Special attention was paid to a possible promotion of reentrant excitation. The drug did not increase the degree of inhomogeneity of conduction and because of the marked effect on refractoriness prevent fast rates (either spontaneously or driven). If the extracellular potassium concentration was lowered (from 5.6 to 2.0 mM), the enhancement of refractoriness by the drug was more marked. The changes in refractoriness do not appear to be based on lengthening of the repolarization of the action potential, but rather on post-repolarization refractoriness; therefore, the occurrence of triggered activity (early afterdepolarizations) is not anticipated. ORG 30701 is a cardioactive drug that because of its calcium antagonistic action acts as an antianginal drug and, in addition, exhibits antiarrhythmic properties.