AbstractUbiquinone (UQ) reductase responsible for reduction of non‐mitochondrial UQ was investigated in rats toward demonstrating an antioxidant role of UQ. In the liver, most of cellular UQ‐10 reductase activity was attributable to a novel NADPH‐UQ reductase in cytosol. The enzyme was not inhibited by dicumarol and rotenone, and had a Km of 19 μM for NADPH and 307 μM for NADH at the optimum pH 7.4. The enzyme was purified 300‐fold to apparent homogeneity from the liver cytosol by an affinity chromatographic method. The purified enzyme reduced UQ‐10 in lecithin liposomes, and protected the liposomes from lipid peroxidation. Furthermore, supplementation of rats with UQ‐10 was observed to increase the enzyme level in their livers without affecting levels of other antioxidant factors. The observations suggested that cytosolic NADPH‐UQ reductase is responsible for cellular UQ redox cycle as an endogenous antioxidant.