Drug-metabolizing activities, especially the reductase activities towards N-oxide, hydroxamic acid, sulfoxide and nitro compounds were comparatively examined with bovine ciliary body. As described previously, the cytosol from the ocular tissue exhibits the nicotinamide N-oxide reductase activity when supplemented with 2-hydroxypyrimidine, an electron donor of aldehyde oxidase. When the cytosol was fractionated with ammonium sulfate, followed by assays of aldehyde oxidase and nicotinamide N-oxide reductase activities in each fraction, the distribution of aldehyde oxidase activity in the resultant ammonium sulfate fractions was nearly parallel to that of nicotinamide N-oxide reductase activity. Furthermore, reductase activities towards drugs such as sulfoxide, hydroxamic acid and nitro compounds were observed with the cytosol in the presence of 2-hydroxypyrimidine or N1-methylnicotinamide. In general, these reductase activities of the fraction were markedly inhibited by menadione, an inhibitor of aldehyde oxidase. These results suggest that aldehyde oxidase present in ciliary body plays an important role in the reduction of a variety of xenobiotics in mammalian eyes. However, in the case of imipramine N-oxide, its reduction in the ocular tissue appears to be more readily catalyzed by a menadione-linked enzyme different from aldehyde oxidase.