Although the KATP channel has been demonstrated to be involved in ischemic preconditioning (IPC) in most species, controversy still exists as to the role of this channel as a mediator of PC in the rat heart. Previously, the authors' laboratories have shown that glibenclamide blocks IPC in the intact rat heart, in a time-dependent manner; however, since glibenclamide has been shown to have non-selective effects unrelated to KATP channel blockade, a structurally dissimilar and ischemia-selective KATP channel blocker, 5-hydroxydecanoate (5-HD), was used to further elucidate the role of KATP channels in mediating IPC in the rat heart. Anesthetized, open-chested Sprague-Dawley rats were subjected to one of four protocols. In Group I, control (C), rats were subjected to 30 min of left coronary artery occlusion and 90 min of reperfusion. In Group II, IPC was elicited by 1 x 5 min occlusion followed by 10 min of reperfusion, prior to the 30 min occlusion and 90 min reperfusion periods, 5-HD (5 mg/kg, i.v.) was given 15 min prior to the 30 min occlusion period in non-preconditioned animals, or given 15 min prior to IPC (5-HD+IPC) (Groups III and IV, respectively). Infarct size (IS), as a percentage of the area at risk (AAR), was determined by triphenyltetrazolium staining. Ischemic preconditioning produced a marked reduction in infarct size (47.5 +/- 3.8% to 7.9 +/- 1.9%, *P < 0.01), which was completely abolished by 5-HD (50.5 +/- 2.6%). These data further suggest that the opening of KATP channels is an important component of IPC in the intact rat heart, similar to that observed in other species.