Intracellular electrophysiological studies were performed on isolated canine cardiac tissues to investigate further the reported ability of clofilium (3 X 10(-8)--10(-6) M) to selectively increase action potential duration (APD) and refractoriness. In Purkinje fibers from normal dogs, clofilium did not influence (1) the rate of rise of the action potential (Vmax) elicited from normal or depolarized (10 mM potassium) resting potentials, (2) the Vmax of premature potentials elicited during the repolarization phase of a previous action potential or (3) the rate of diastolic depolarization of spontaneously firing Purkinje fibers. The diastolic interval was altered by inserting a single premature impulse during diastole or by varying the basic cycle length. Clofilium (3 X 10(-7) M) slightly reduced the time constant for the relation between diastolic interval and APD in concentrations that caused a maximal increase in APD of nonpremature impulses. In dogs subjected to occlusion of the left anterior descending coronary artery 48 hr before study, the APD of surviving Purkinje fibers was longer in the infarcted zone than in the normal zone. Clofilium (3 X 10(-8) M) increased APD in both zones but more so in the normal areas, thus reducing the disparity of APD between zones. Similarly, clofilium (3 X 10(-8) and 3 X 10(-7) M) increased the effective refractory period in both zones but more so in the normal area. The increase of APD and refractoriness in normal as well as depolarized or ischemic tissues in the absence of marked changes in Vmax and conduction may decrease the likelihood of reentrant arrhythmias and underlie the antifibrillatory effects in anesthetized dogs.