The objective of this study was to identify cellular electrophysiological mechanisms by which ischemic preconditioning decreases arrhythmias in an isolated ventricular tissue model of ischemia and reperfusion. Electrical activity was recorded with microelectrodes from endocardium and epicardium of paced guinea pig right ventricular free walls. Control preparations were exposed for 15 min to Tyrode solution modified to simulate selected ischemic conditions and then were reperfused for 30 min with normal solution. Preconditioned tissues were exposed to a 2- or 5-min period of simulated ischemia before this same protocol. Neither preconditioning protocol affected incidence of ventricular tachycardia (VT) in ischemia; however, the 5-min protocol significantly decreased premature beats (PVB) and transmural conduction block. Preconditioning for 5 min, but not 2 min, significantly decreased reperfusion-induced VT and PVB. Ischemic preconditioning did not change effects of ischemia or reperfusion on action potential duration, effective refractory period, or endocardial conduction time. However, preconditioning markedly attenuated depression of transmural conduction by ischemia and early reperfusion and thereby prevented conduction delays necessary for transmural reentry.