Aldose reductase (alditol:NADP oxidoreductase EC .1.1.1.21), an enzyme in the sorbitol pathway which has been implicated in the pathogenesis of diabetic complications, has been purified from rat lens (RLAR) by affinity chromatography with Amicon Matrex Gel Orange A and its properties have been compared to those of purified human placental aldose reductase (HPAR). The RLAR appears to be closely associated with alpha- and beta-crystallin and has a higher affinity for the dye Matrex column than HPAR. The purified enzyme, obtained upon elution from the column, appears as a closely-spaced doublet of approximately 38 K MW on SDS-PAGE which does not immunologically cross-react with antibodies raised against the single 38 K MW HPAR. Antibodies raised against RLAR however do cross-react with HPAR. Kinetic studies indicated both enzymes to have a greater apparent affinity for aliphatic and aromatic aldehydes than for aldose sugars. Compared to DL-glyceraldehyde, RLAR displayed an 80-fold greater affinity for p-nitrobenzaldehyde and a 1000-fold decreased affinity for D-glucose while HPAR displayed a 15-fold greater affinity for p-nitrobenzaldehyde and 600-fold less affinity for D-glucose. Both enzymes displayed only trace activity with 200 mM L-gulonic acid.