Lorcainide is a new antiarrhythmic agent which effectively suppresses ventricular premature contractions. Its electrophysiological actions were studied in 15 cardiac patients with and without cardiac conduction abnormalities. In a dose of 2 mg/kg body weight, lorcainide decreased the sinus cycle length (SCL) and increased the corrected sinus node recovery time. The intra-atrial, atrioventricular (AV), and intraventricular conduction times were prolonged. A third-degree AV block developed in 2 patients with pre-existing conduction abnormalities. QRS widening showed that intramyocardial conduction was also affected. The effective refractory period of the atrioventricular node (ERP-AVN) was shortened, whereas the effect on the functional refractory period of the AVN was variable. The decrease in SCL and ERP-AVN may be due to a possible anticholinergic effect of the drug. The accessory pathway was blocked in 3 patients with a Wolff-Parkinson-White syndrome. The ERP of the ventricle was slightly prolonged. The electrophysiological profile of the drug, i.e., slowing of conduction velocity throughout the heart combined with shortening of SCL and ERP-AVN, differs from other antiarrhythmic agents.