Photolytic decomposition of sodium nitroprusside (SNP), a widely used nitrovasodilator, produced nitric oxide (.NO), which was continuously monitored by electron spin resonance (ESR) spectroscopy. The .NO present in the aqueous or the lipid phase was trapped by either a hydrophilic or a hydrophobic nitronyl nitroxide, respectively, to form the corresponding imino nitroxide. The conversion of nitronyl nitroxide to imino nitroxide was monitored by ESR spectrometry. The quantum yield for the generation of .NO from SNP, measured from the rate of decay of nitronyl nitroxide, was 0.201 +/- 0.007 and 0.324 +/- 0.01 (mean +/- SD, n = 3) at 420 nm and 320 nm, respectively. The action spectrum for .NO generation was found to overlap the optical absorption spectrum of SNP closely. A mechanism for the reaction between SNP and nitronyl nitroxide in the presence of light is proposed and computer-aided simulation of this mechanism using published rate constants agreed well with experimental data. The methodology described here may be used to assay .NO production continuously during photoactivation of .NO donors in aqueous and lipid environments. Biological implications of this methodology are discussed.