Heme oxygenase (HO) is a heme-catabolizing enzyme that converts heme into biliverdin, iron, and carbon monoxide. HO-1, an inducible form of HO, is thought to act as an endogenous antioxidant defense mechanism. To determine whether chronic administration of angiotensin II affects HO-1 expression in the heart, expression and localization of HO-1 were investigated in the heart of rats receiving angiotensin II infusion (0.7 mg. kg(-1). day(-1)) via osmotic minipump for up to 7 days. Angiotensin II induced formation of granulation tissue, characterized by myofibroblast proliferation, fibrous deposition, and inflammatory cell migration. Angiotensin II also upregulated cardiac HO-1 expression. Immunohistochemistry revealed that HO-1 was intensively expressed in the granulation tissue. The selective AT(1)-receptor antagonist, losartan, completely, but hydralazine only partially, suppressed angiotensin II-induced granulation tissue formation and HO-1 upregulation. Chronic norepinephrine infusion (2.8 mg. kg(-1). day(-1)) did not induce granulation tissue formation or HO-1 upregulation. Our data suggest that angiotensin II upregulates cardiac HO-1 expression in the newly formed inflammatory lesion, which may represent an adaptive response to angiotensin II-induced cardiac damage.