Terikalant is a benzopyran derivative patented by Rhone-Poulenc Sante as an antiarrhythmic agent. In normal cardiac tissues studied in vitro, Terikalant dose-dependently prolonged the atrial and ventricular action potential but affected neither the upstroke of the action potential nor the diastolic potential. Patch-clamp experiments demonstrated that the prolongation of the action potential induced by Terikalant is due to a specific blockade of the inward rectifier K+ current. In vivo, i.v. administration to anesthetized dogs of low doses of Terikalant consistently induced bradycardia and prolonged the atrial, nodal, and ventricular refractory periods, but did not affect the conduction velocity.