Nitronyl nitroxides, NN . , have been increasingly used in the field of NO-related studies as specific antagonists of NO . . We employed a combination of EPR and NMR spin trapping to study the mechanisms of the reaction of NN . with NO . in reducing environments. EPR allowed observation of NO-induced transformation of the paramagnetic trap, NN . , to the corresponding iminonitroxide, IN . . In a complementary way, corresponding EPR-invisible diamagnetic products (the hydroxylamines NN-H and IN-H) were detected by 1 9 F-NMR using newly synthesized fluorinated traps. Addition of reducing agents to a solution of NN . resulted in fast disappearance of its EPR spectra and appearance of a 1 9 F-NMR peak of the corresponding hydroxylamine, NN-H. Addition of NO . as a bolus, or NO . generated on sodium nitroprusside photolysis, resulted in 1 9 F-NMR-detectable accumulation of the hydroxylamine, IN-H. Upon high rates of NO . generation in ascorbate-containing solutions, partial recovery of NN . was observed, which undergoes further reactions with NO . and ascorbate in a competitive manner. Using 1 9 F-NMR and a fluorinated trap, NO-induced conversion of NN-H into IN-H was also observed in vivo in hypertensive ISIAH rats compared with normotensive WAG rats. The results provide insight into a new potential redox-sensitive mechanism of the antagonistic action of NN . against NO . , which may provide insight into previously unexplained behavior of this category of NO-reacting compounds.