Background—Lengthening of the QT interval and torsades de pointes resulting in cardiac arrests and deaths have been noticed during treatment with cisapride, a newly developed gastrointestinal prokinetic agent. The rapid (IKr) and slow (IKs) components of the delayed rectifier current (IK) are candidate ionic currents to explain cisapride-related toxicity because of their role in repolarization of cardiac ventricular myocytes. Our objectives were to (1) characterize effects of cisapride on two major time-dependent outward potassium currents involved in the repolarization of cardiac ventricular myocytes,IKrandIKs, and (2) determine action potential–prolonging effects of cisapride on isolated hearts.Methods and Results—A first set of experiments was performed in isolated guinea pig ventricular myocytes with the whole-cell configuration of the patch-clamp technique. Cells were held at −40 mV while time-dependent outward currents were elicited by depolarizing pulses lasting either 250 ms (IK250) or 5000 ms (IK5000). Effects of cisapride on theIKrcomponent were assessed by measurement of time-dependent activating currents elicited by short pulses (250 ms;IK250) to low depolarizing potentials (−20, −10, and 0 mV). Time-dependent activating currents elicited by long pulses (5000 ms;IK5000) to positive potentials (>+30 mV) were recorded to assess effects of the drug on theIKscomponent. A second set of experiments was conducted in isolated guinea pig hearts buffer-perfused in the Langendorff mode to assess effects of the drug on monophasic action potential duration measured at 90% repolarization (MAPD90). Hearts were exposed to cisapride 100 nmol/L at decremental pacing cycle lengths of 250, 225, 200, 175, and 150 ms to determine reverse frequency-dependent effects of the drug. Overall, 112 myocytes were exposed to seven concentrations of cisapride (10 nmol/L to 10 μmol/L). Cisapride inhibitedIKr, the major time-dependent outward current elicited by short pulses (IK250) to low depolarizing potentials, in a concentration-dependent manner with an IC50of 15 nmol/L (therapeutic levels, 50 to 200 nmol/L). Conversely, block ofIKsby the drug was less potent (estimated IC50>10 μmol/L). In isolated hearts (n=9 experiments), cisapride 100 nmol/L increased MAPD90by 23±3 (P<.05) at a basic cycle length of 250 ms but by only 7±1 ms (P<.05) at a basic cycle length of 150 ms.Conclusions—Block ofIKrgives an explanation to lengthening of cardiac repolarization observed in isolated guinea pig hearts. Potent block ofIKris also likely to underlie prolongation of the QT interval observed in patients receiving clinically recommended doses of cisapride as well as severe cardiac toxicity (torsades de pointes) observed in patients with increased plasma concentrations of the drug.