Genetic and pharmacological inhibition of inducible nitric oxide synthase (iNOS) decreases atherosclerosis development. Potential proatherogenic effects of iNOS include iNOS mediated oxidative stress and iNOS expression in different cellular compartments. Lesional iNOS can potentially produce nitric oxide radicals (NO), superoxide radicals (O2(-)), or both; these radicals may then react to form peroxynitrite. Alternatively, O2(-) radicals from oxidases co-expressed with iNOS could react with NO to produce peroxynitrite. Therefore, the expression profiles of the genes that modulate the redox system in different iNOS-expressing cell compartments may determine the role of iNOS in atherosclerosis. We used apoE (apoE(-/-)) and apoE/iNOS double knockout (apoE(-/-)/ iNOS(-/-)) mice to assess vascular NO, O2(-), and peroxynitrite formation by electron spin resonance spectroscopy, high performance liquid chromatography, and 3-nitrotyrosine staining. The relevance of the iNOS expressing cell compartment was tested by bone marrow transplantation. We show that iNOS significantly contributes to vascular NO production and itself produces O2(-), leading to peroxynitrite formation in atherosclerotic lesions. Our bone marrow transplantation experiments show that bone marrow derived cells exclusively mediate the proatherosclerotic effects of iNOS in males, while both parenchymal and bone marrow derived iNOS equally contribute to atherosclerosis in females. Moreover, iNOS expression affects vascular remodeling. These findings establish for the first time that the proatherosclerotic effects of iNOS vary with sex in addition to the compartment of its expression.