The effects of a series of dicarboxylic acids on aldose reductase activity were investigated in a crude enzyme preparation obtained from ocular lenses of the swine. Of the compounds examined, maleic acid inhibited aldose reductase in a pronounced manner and in a concentration-dependent fashion (IC50 value of 83.0 +/- 1.4 mM). Saturation experiments revealed non-competitive kinetics with a Ki value of 64.9 +/- 1.7 mM. The present results suggest that, at higher doses, maleic acid might have some implication for the treatment of diabetic complications.