Ischemic preconditioning (IPC) confers cardioprotection against a prolonged ischemic insult. Tyrosine kinase (TK) inhibitors have been shown to attenuate IPC; however, it is unclear whether TK is involved in the initiation of and/or the maintenance of this phenomenon. Thus the hypothesis that TK acts primarily during the initiation of IPC was examined in a rat model of myocardial infarction. Hearts were subjected to 30 min of coronary artery occlusion and 2 h of reperfusion. IPC was elicited by three 5-min occlusions interspersed with 5 min of reperfusion before the prolonged occlusion period. Genistein, a nonspecific TK inhibitor, was administered before or during the final 2 min of the first or third occlusion period of IPC. Daidzein, an inactive structural analog of genistein, and lavendustin A, a more specific TK inhibitor, were also tested in this model. IPC markedly reduced infarct size expressed as a percentage of the area at risk compared with control (56.3 +/- 2.8 to 7.1 +/- 2.0%). This cardioprotection was attenuated by genistein pretreatment (5 mg/kg: 34.7 +/- 2.2%, 10 mg/kg: 33.5 +/- 5.9%). However, genistein administered during the first or third occlusion period of IPC did not significantly attenuate cardioprotection (10.3 +/- 2.9% and 6.4 +/- 2.0%). Lavendustin A (1. 0 mg/kg) pretreatment also attenuated IPC (30.1 +/- 2.2%), whereas daidzein (5 mg/kg) had no effect (7.9 +/- 2.4%). These results suggest that activation of a TK is involved in the initiation but not the maintenance of IPC in the rat myocardium.