The present study provides the evidence that liver aldehyde oxidase in the presence of its electron donors can catalyze the reduction of N-hydroxyurethane to urethane under anaerobic conditions. Guinea pig liver 9000 X g supernatant and cytosol, but not liver microsomes, exhibited N-hydroxyurethane reductase activity in the presence of acetaldehyde or 2-hydroxypyrimidine. The cytosolic enzyme was precipitated with ammonium sulfate between 30 and 45% ammonium sulfate saturation. The N-hydroxyurethane reductase and aldehyde oxidase activities of the precipitate were similarly susceptible to inhibition by a variety of chemicals. When the precipitate was chromatographed on a DEAE-cellulose column, the elution peak position of N-hydroxyurethane reductase was entirely identical with that of aldehyde oxidase. Furthermore, purified rabbit liver aldehyde oxidase also exhibited a significant N-hydroxyurethane reductase activity in the presence of acetaldehyde or 2-hydroxypyrimidine.