Aldose reductase (AR) was purified from rat seminal vesicles. Specific antibodies to this enzyme were prepared in rabbits and were used in the unlabeled antibody-enzyme (PAP) technique to localize AR in a number of tissues known to be sites of diabetic lesions. AR was localized in the following structures in the eye: lens epithelial lining and cortical lenticular fibers; corneal endothelium; the inner, nonpigmented layer of ciliary body epithelium and its extension as the posterior surface of the iris; and neuroglial (Müller) cells in the retina. Retinal capillary endothelium did not contain immunoreactive AR. In the kidney, staining was intense in the inner medulla. Specific structures included thin limbs of the loop of Henle, collecting tubules deep in the inner medulla, and transitional epithelial cells lining the pelvic space: structures in the cortex including glomerular podocytes and distal convoluted tubules. Collecting tubules in the outer medulla and cortex, as well as proximal convoluted tubules and glomerular capillary endothelium did not contain immunoreactive AR.