Recent results have shown that the sulfonylurea receptor couples to several types of inward-rectifier potassium (K_IR) channels, which suggests that sensitivity to blockade of a pathophysiological phenomenon such as ischemic preconditioning (PC) by glibenclamide may not be the result of this compound selectively blocking the ATP-sensitive potassium (K_ATP) channel. Therefore, to address this possibility, a role for myocardial K_IRvK_ATPchannels in ischemic PC was evaluated in the rat. To test this hypothesis, anesthetized, open-chest, male Wistar rats were assigned to one of seven experimental protocols. Animals assigned to group I (control) received 30 min of occlusion and 2 h of reperfusion. Ischemic PC was produced by 3×5-min occlusion and 2-h reperfusion periods (group II). Terikalant (TK), an inward-rectifier potassium channel blocker, was used to test the role of other K⁺channels, most notably the K_IR, in the cardioprotective effect of ischemic PC in the rat. TK was given at a dose of 3 mg/kg, i.v., 15 min before the prolonged occlusion and reperfusion periods (group III). In groups IV, V, and VI terikalant (1, 3 and 6 mg/kg, i.v.) was given 15 min before ischemic PC (lowTK+PC, medTK+PC and hiTK+PC, respectively). Group VII consisted of glibenclamide (0.3 mg/kg, i.v.) given 30 min prior to ischemic PC (GLY+PC). Infarct size (IS) as a percent of the area at risk (AAR) was measured using the histochemical stain, 2,3,5-triphenyltetrazolium chloride. The average IS/AAR for the control was 49.9±2.1%. Ischemic PC markedly reduced infarct size (8.6±1.8%; *P<0.05vcontrol). Terikalant (TK; 1, 3 and 6 mg/kg, i.v.) did not abolish the cardioprotective effect of ischemic PC at any dose (15.5±6.4, 16.4±5.2 and 8.8±1.6%, respectively; *P<0.05vcontrol). TK itself had no effect on infarct size. GLY completely abolished the cardioprotective effect of ischemic PC (48.2±6.4%). In addition, the high dose of TK significantly (P<0.05) increased the action potential duration at 50% repolarization from 48±3 to 64±4 ms and 30μmof TK, a concentration which produced a 39% decrease in the inward-rectifier potassium channel current in isolated guinea-pig ventricular myocytes in the whole-cell patch-clamp mode did not block the increase in K_ATPcurrent produced by the K_ATPopener bimakalim (3μm). These results demonstrate that although the myocardial K_ATPchannel belongs to the K_IRsuperfamily, the endogenous myocardial K_IRchannel does not mediate ischemic PC in the rat heart; however, the K_ATPchannel does mediate its cardioprotective effect.