Aldehyde reductase (E.C. 1.1.1.2) has been purified to apparent homogeneity from rat lens and its properties have been compared to those of both rat lens aldose reductase and rat kidney aldehyde reductase. The purification was accomplished by ammonium sulfate fractionation, Sephadex G-75 chromatography, affinity chromatography on Amicon Matrex Gel Orange A and chromatofocusing. The purified enzyme is distinct from rat lens aldose reductase but appears similar to rat kidney aldehyde reductase in molecular weight, immunological properties and substrate specificities. Rat lens aldehyde reductase is inhibited by a number of aldose reductase inhibitors; however, its susceptibility to inhibition is more similar to rat kidney aldehyde reductase than to rat lens aldose reductase.