Background The aim of this study was to assess whether and how infarct size (IS) reduction by postconditioning is modified in the heart with coronary stenosis (CS), and how β-blocker treatment affects it. Methods and Results The IS was assessed by 30-min acute coronary occlusion and 24-h reperfusion in rat hearts in which CS had been induced. Modification of IS by postconditioning and the effects of daily carvedilol treatment were measured, together with reperfusion injury - salvage kinase activities. Four weeks after CS induction, any reduction of IS by postconditioning was lost, whereas it reduced IS in rats without CS. In the hearts without CS, postconditioning activated both the ERK and Akt pathway. However, in the hearts with CS, postconditioning failed to do so. In hearts with CS plus daily carvedilol treatment, postconditioning reduced IS compared with the hearts without carvedilol, although postconditioning did not activate the Akt and ERK pathways. Conclusion CS impaired Akt and ERK activation, resulting in a failure to reduce IS by postconditioning. Carvedilol treatment restored the IS reduction by postconditioning, possibly via other mechanism(s) of the ERK and Akt pathways. (Circ J 2008; 72: 2081 - 2086)