Plasma nitrite/nitrate levels reflect oxidation of formed nitric oxide (NO ) but are not indicative of endothelial nitric oxide synthase (eNOS) function due to interference by dietary nitrates and reactive oxygen species (ROS). Nitrosyl hemoglobin (NOHb), a metabolic product of nitric oxide, may better correlate with bioavailable NO but it may depend on the activity of different nitric oxide synthase (NOS) isoforms and may be affected by dietary nitrite/nitrate. We examined the correlation between vascular endothelial NO release and circulating blood levels of NOHb. We measured NOHb in blood using electron spin resonance (ESR) spectrometry and also quantified vascular production of NO using colloid Fe(DETC)(2) and ESR in mouse and human venous blood before and after treatment with the beta-blocker carvedilol. Exclusively the inhibition with L-NAME and not the treatment with the selective neuronal nitric oxide synthase (nNOS) inhibitor, N-AANG or with the selective inducible nitric oxide synthase (iNOS) inhibitor, 1400W, halved NOHb formation, which reflects the complete inhibition of NO release by aortic endothelium. The relationship between NOHb and NO production by the endothelium (0.23 microM NOHb to 3.73 microM/hour of NO per mg of aorta dry weight) was found to be identical for both C57Blk/6 mice and for mice with vascular smooth muscle-targeted expression of p22phox associated with strong increase in eNOS activity. Furthermore, the treatment of patients with cardiovascular diseases with carvedilol for 3 weeks increases up to 2 times the circulating NOHb concentration. These results demonstrate the important role of eNOS in the formation of circulating NOHb and suggest that NOHb can be used as a noninvasive marker of endothelial NO production in vivo.