The action of aldose reductase has been implicated in the etiology of a variety of diabetic complications affecting the visual system. However, very little is known regarding the structure and functional organization of the genes encoding this key enzyme. In the present study, we have isolated and characterized complementary DNA clones encoding bovine lens aldose reductase. Nucleotide sequencing of four independently isolated clones was used to establish a 1154 nucleotide composite cDNA sequence. The cDNA sequence encodes 296 amino acids of the aldose reductase primary structure, and contains an additional 261 nucleotides of apparently untranslated sequence downstream from the coding region. No nucleotide sequence differences were found among the four independently isolated aldose reductase cDNA clones. The aldose reductase amino acid sequence deduced from the cDNA shows high homology to that reported for aldose reductase of the rat lens. Significant similarities are also evident between bovine lens aldose reductase and both human liver aldehyde reductase and frog lens rho-crystallin.