Nitroxide radicals (nitroxides) are reduced to the corresponding hydroxylamines and lose their electron spin resonance (ESR) signals, but these hydroxylamines are easily reoxidized to nitroxides and regain the ESR signals. In the present study the effects of nitric oxide (NO) on the reduction/oxidation (redox) status of hepatic microsomes were investigated by ESR spectroscopy using nitroxide probes. Rat hepatic microsomes were treated with an NO donor, NOR3 or NOC7, and then labeled with a water-soluble nitroxide, 2,2,6,6-tetramethyl-4-hydroxy-1-piperidinyloxy (Tempol), or a lipid-soluble nitroxide, 5-doxyl stearic acid (5-DSA). The reduction of Tempol was facilitated under hypoxic conditions in control microsomes. In NOR3 or NOC7-treated microsomes, the reduction of Tempol and the reoxidation of the corresponding hydroxylamine hardly occurred under both normoxic and hypoxic conditions. The ESR signals of 5-DSA changed just as those of Tempol did in control and NO-treated microsomes. The concentrations of total thiol and cytochrome P-450, and the activity of mixed function amine oxidase were reduced in NOR3 or NOC7-treated microsomes. In conclusion, NO affects not only the reduction of nitroxides but also the oxidation of hydroxylamines in hepatic microsomes, suggesting that the microsomal capability of redox regulation was lost by NO.