While endothelial nitric oxide synthase has been reported to be expressed in the endothelial cells of normal and atherosclerotic vessels, there are few reports about inducible nitric oxide synthase (iNOS). We investigated the expression of iNOS and its relation to inflammatory cells in atheroma. New Zealand White rabbits were fed 1 of 4 diets: (i) normal diet for 9 weeks; (ii) normal plus 1% cholesterol diet (atherogenic diet) for 9 weeks; (iii) atherogenic diet for 9 weeks, then normal diet for 9 weeks; (iv) atherogenic diet for 9 weeks, then the normal diet for 36 weeks. The aortas were examined by immunohistochemical staining for anti-iNOS antibody, as well as antibodies for macrophages, T lymphocytes, and muscle actin. No iNOS was detected in normal aortas, intimal thickenings, or fatty streaks. Although iNOS was detected in necrotic cores of advanced plaque, it was not seen in smooth muscle-derived cells or endothelial cells but was found in some macrophage-derived cells and in T lymphocytes. In regressive atherosclerotic aortas, iNOS was detected only in necrotic cores, not in macrophage-derived cells but in T lymphocytes. These findings suggest that T lymphocytes and some macrophages induce iNOS through cytokine production in atheroma. This is the first report of iNOS expression in atheromatous plaque.