The role for peroxynitrite (ONOO-) in the mechanism of preconditioning is not known. Therefore, we studied effects of preconditioning and subsequent ischemia/reperfusion on myocardial ONOO- formation in isolated rat hearts. Hearts were subjected to a preconditioning protocol (three intermittent periods of global ischemia/reperfusion of 5 min duration each) followed by a test ischemia/reperfusion (30 min global ischemia and 15 min reperfusion). When compared to nonpreconditioned controls, preceding preconditioning improved postischemic cardiac performance and significantly decreased test ischemia/reperfusion-induced formation of free nitrotyrosine measured in the perfusate as a marker for cardiac endogenous ONOO- formation. During preconditioning, however, the first period of ischemia/reperfusion increased nitrotyrosine formation, which was attenuated after the third period of ischemia/reperfusion. We conclude that classic preconditioning inhibits ischemia/reperfusion-induced cardiac formation of ONOO- and that subsequent periods of ischemia/reperfusion result in a gradual attenuation of ischemia/reperfusion-induced ONOO- generation. This mechanism might be involved in ischemic adaptation of the heart.