The electron spin resonance signal of Tempol decays in the presence of red cells. The decay is due to reduction of oxidant, paramagnetic nitroxide group by the metabolic activity of the red cell. In normal red cells, GSH level was stable and Tempol reduction rate followed a first-order kinetics. In G6PD-deficient red cells, GSH dropped and Tempol reduction rate was slower and followed a second-order kinetics. In normal red cells, diamide reversibly oxidized GSH. First-order kinetics of Tempol reduction rate was attained after a delay time proportional to the diamide concentration and corresponding to the full regeneration of GSH. In diamide-treated G6PD deficient, and in NEM-treated, normal red cells, irreversible disappearance of GSH was followed by irreversible dose-dependent decrease in Tempol reduction rate. A correlation between GSH levels and Tempol reduction rate was observed. A correlation was also established between Tempol reduction rate and stimulation of pentosephosphate shunt activity.