Nitric oxide (.NO) exerts a major regulatory physiological role in the cardiovascular and central nervous system. .NO is also metabolically released from nitrovasodilators such as nitroprusside, nitroglycerin, and synonimines whose mechanism of cardioprotection appears to be related to their .NO release. However, techniques to detect and quantitate .NO in solution are limited. In this paper, we report an electron spin resonance (ESR) technique that will continuously monitor the production of .NO in solution. This methodology is based on the reaction between .NO and nitronyl nitroxides (a group of organic compounds with nitronyl and nitroxide functional groups) to produce imino nitroxides. The ESR spectra of nitronyl nitroxides and imino nitroxides are characteristic and distinctly different. Nitronyl nitroxide to imino nitroxide conversion is catalyzed only by .NO, but not by superoxide anion, hydroxyl radical or nitrogen dioxide (.NO2). Nitrite ion (NO2-) was determined to be a major product of this reaction. Using ESR and optical techniques, we have continuously monitored the production of .NO and NO2- during visible irradiation of sodium nitroprusside. We conclude that this ESR approach can be applied to probe mechanisms of .NO and NO2- production from nitrovasodilators. The clinical implications of trapping of .NO by nitronyl nitroxides are also discussed.