The concentration of nitrate plus nitrite, metabolic end products of nitric oxide, in serum prepared from systemic venous blood was significantly (p <0.001) decreased in both heavy (14.5 +/- 1.3 micromol/L) and moderate (17.6 +/- 2.3 micromol/L) smokers relative to that in nonsmokers (22.6 +/- 0.4 micromol/L).