Background - Heme oxygenase-1 (HO-1) is proposed to have a variety of adaptive responses against oxidative stress. To examine the function of HO-1 against atherogenesis in vivo, we observed the effects of HO-1 inhibition on atherosclerotic lesion formation in Watanabe heritable hyperlipidemic rabbits (WHHL). Methods and Results - During 4 weeks of a 1% cholesterol diet, intravenous injections of Sn-protoporphyrin IX to inhibit HO-1 (S group, n=10) and saline as a control (C group, n=10) were given to 3-month-old WHHL rabbits. The percentages of en face atherosclerotic lesion areas in total descending aorta by Sudan IV staining (EFA) and the ratio of intima to media in microscopic atherosclerotic lesions in the ascending aortas (I/M) were calculated. Two different quantitative methods revealed significantly greater atherosclerotic lesions in the S group than the C group (EFA, P<0.001; I/M, P<0.005). HO-1 expression in atherosclerotic lesions was confirmed by Northern blot and immunohistochemical analyses. The dominant cell types expressing HO-1 were macrophages and foam cells, in which oxidized phospholipids were also accumulated. HO inhibition increased plasma and tissue lipid peroxide levels without affecting plasma lipid composition. Conclusions - These results suggest the possibilities that HO-1 has antiatherogenic properties in vivo and that the antiatherogenic properties of HO-1 are conducted through the prevention of lipid peroxidation.