Four aldehyde reductases, F1, F2, F3, and F4, were isolated from rabbit liver cytosol to homogeneity by various chromatographic techniques. F2 is an aldehyde reductase with a molecular weight of 32,000, which resembled aldehyde reductases of human liver and pig kidney in properties. It was inhibited in a noncompetitive way by alpha-ketoglutarate and oxaloacetate with Ki values of 4 x 10(-5) M. The other three enzymes were NADPH-dependent aromatic aldehyde-ketone reductases. F1 and F3 were monomeric enzymes with molecular weights of 38,000 and 29,000, respectively. F4 showed a molecular weight of 78,000 on gel filtration, but sodium dodecyl sulfate gel electrophoresis revealed two different subunits with molecular weights of 26,000 and 24,000. The molar ratio of NADPH : F1, F2, or F3 binding was 1 : 1, whereas that of NADPH:F4 binding was 3 : 1. The number of thiol groups in order molecule of F1, F2, F3, and F4, was 4, 4, 4, and 5, respectively. The enzyme activity of F3 was inhibited by addition of an equal mole amount of PCMB. Only F4 was inhibited by metal chelating agents. F1 also catalyzed the interconversion of 3(17)-keto and 3(17) beta-hydroxysteroids, whereas F3 catalyzed oxidoreduction of some 3 alpha-hydroxysteroids of the 5 alpha-series. These results suggest that F1 and F3 are 3(17) beta-hydroxysteroid dehydrogenase and 3 alpha-hydroxysteroid dehydrogenase, respectively. Endogenous substrates for F4 could not be identified in this work. F1 showed the lowest Km value for reduction of aldehydes and ketones among the four reductases. It has been suggested that F1 is the primary enzyme responsible for the reduction of endogeneous aldehydes and xenobiotic aldehydes and ketones in vivo.