The class III agents d-sotalol and dofetilide have been shown to exhibit differential effects in large controlled clinical trials. The aim of this study was to investigate the basic electrophysiological properties of these two antiarrhythmia agents in an in vitro experimental model with regard to potential antiarrhythmic and proarrhythmic action. Using standard microelectrode techniques, we evaluated the electrophysiological effects of d-sotalol and dofetilide on action potential parameters recorded from guinea pig papillary muscle at 2.5 mM, 3.5 mM, and 5.6 mM extracellular potassium concentrations. The following parameters were recorded: resting membrane potential (RMP), action potential amplitude (APA), action potential duration at 90% repolarization (APD 90), and maximum upstroke velocity (Vmax). Under all conditions studied, both d-sotalol and dofetilide exhibited highly selective reverse rate-dependent class III action. In contrast to dofetilide, the class III activity of d-sotalol was markedly influenced by changes in extracellular potassium concentrations, predominantly at low pacing rates. Hypokalemia enhanced the action potential-prolonging effects of d-sotalol, whereas hyperkalemia diminished this effect. In addition, reverse rate dependence associated with dofetilide was significantly more pronounced than reverse rate dependence associated with d-sotalol. Our observations provide a potential electrophysiological basis for differential antiarrhythmic and proarrhythmic mechanisms associated with these two drugs.