Two NADPH-linked aldehyde reductases (alcohol:NADP+ oxidoreductase, EC 1.1.1.2), referred to here as aldehyde reductases I and II, have been purified to homogeneity from human liver by using ammonium sulfate precipitation, ion-exchange chromatography, affinity chromatography and gel filtration. Structural studies show that aldehyde reductase II is a monomer of about 32 000 daltons, whereas aldehyde reductase I is a dimer of two nonidentical subunits of molecular weights about 42 000 and 35 000. The isoelectric pH was determined to be 5.40 for aldehyde reductase II and 8.25 for aldehyde reductase I. Substrate specificity studies show that neither aldehyde reductase I nor II uses glucose as substrate but that both are capable of reducing various other aldehydes such as pyridine 3-aldehyde, butyraldehyde and DL-glyceraldehyde. The pH optimums for aldehyde reductases I and II are pH 6.0 and 7.0 respectively. Aldehyde reductase I uses both NADH and NADPH as cofactor, whereas aldehyde reductase II activity is dependent on NADPH. Aldehyde reductase I activity is more susceptible than aldehyde reductase II activity to inhibition by p-hydroxymercuribenzoate, as reflected by IC50 values of 7.5 microM and 40 microM for aldehyde reductases I and II, respectively. The susceptibility of human liver aldehyde reductases I and II to inhibition by the aldose reductase (EC 1.1.1.21) inhibitors 3,3'-tetramethylene glutaric acid, alrestatin, chromone and sorbinil was determined and compared with that of aldose reductase partially purified from bovine lenses. The aldose reductase inhibitors, besides inhibiting aldose reductase, also inhibit human liver aldehyde reductases I and II to varying degrees.