Thirty-three diabetic subjects were given the aldose reductase inhibitor sorbinil (Pfizer, UK) for 3 wk. There was a significant fall in mean erythrocyte sorbitol concentration over this period. In all subjects erythrocyte sorbitol concentrations after treatment were within or below the range found in normal subjects. No changes in erythrocyte 2,3-diphosphoglycerate (2,3-DPG) or myo-inositol concentrations, plasma beta-thromboglobulin (beta-TG) concentration, or P50--a measure of the oxygen affinity of hemoglobin--were observed. There was a high incidence of adverse reactions to the drug.