In the presence of ATP hepatic microsomes sequester calcium. This sequestration is thought to be important in the modulation of free cytosolic calcium concentration. We find that on the addition of NADPH the uptake of calcium by the hepatic microsomes is inhibited 27-85%.This inhibition is reversed by the addition of 1 mm reduced glutathione (85-91% of control), incubation under a nitrogen atmosphere (112% of control), or incubation in a 80% carbon monoxide/20% oxygen atmosphere (75% of control). Superoxide dismutase had no effect on the inhibition, while catalase reversed the inhibition by 35%. The addition of 1 mm reduced glutathione at 2 and 5 min after the addition of NADPH led to uptakes of calcium which paralleled the uptake seen when the reduced glutathione was added at the beginning of the incubation. The effect of reduced glutathione showed saturation kinetics with a Km of 10 μm. Together these data suggest that cytochrome P-450 reduces the activity of the microsomal ATP-dependent calcium pump both by the production of hydrogen peroxide and by the direct oxidation of the protein thiols. The reversal of this effect by reduced glutathione appears to be enzymatically catalyzed.