A long-standing interest of this group is the evaluation of action potential prolonging drugs both at the most basic laboratory level as well as in patients. Sematilide is one of the first of the new "class III' ("pure" action potential prolonging) drugs to become available for clinical evaluation in man. This study evaluates the antiarrhythmic and electrophysiologic, and pharmacokinetic responses to single intravenous doses of sematilide in patients with frequent ventricular ectopic depolarizations.