Oxidative stress is involved in the tolerance to ischemia/reperfusion (I/R) injury. Because angiotensin II type-1 receptor blockers (ARBs) inhibit oxidative stress, it is concerned that ARBs abolish the tolerance to I/R injury. Dhal salt-sensitive (DS) hypertension and salt-resistant (DR) normotension rats received an antioxidant, 2-mercaptopropionylglycine (MPG) or an ARB, losartan for 7 days. Losartan and MPG significantly inhibited oxidative stress as determined by tissue malondialdehyde+4-hydroxy-noneal and increased expression of inducible nitric oxide syntase (iNOS) in the DS rat heart. However, losartan but not MPG activated endothelial nitric oxide synthase (eNOS) as assessed by phosphorylation of eNOS on Ser 1177. Infarct size after 30 minutes left coronary artery occlusion followed by 2 hours reperfusion was comparable between the DS and the DR rat hearts. Although MPG and losartan had no effect on infarct size in the DR rat heart, MPG but not losartan significantly increased infarct size in the DS rat heart. A selective iNOS inhibitor, 1400W increased infarct size in the DS rat heart, but it had no effect on infarct size in the losartan-treated DS rat heart. However, a non-selective NOS inhibitor, N-nitro-L-arginine methyl ester increased infarct size in the losartan-treated DS rat heart. These results suggest that losartan preserves the tolerance to I/R injury by activating eNOS despite elimination of redox-sensitive upregulation of iNOS and iNOS-dependent cardioprotection in the DS rat heart.