Human aorta, brain, and muscle aldose reductase, partially purified by DEAE-cellulose (DE-52) column chromatography, is activated 2–2.5-fold on incubation with 10 μM each of glucose-6-phosphate, NADPH, and glucose for 20 min at 25°C. The activation of the enzyme was established by following the NADPH oxidation as well as the sorbitol formation using glucose as substrate. The activated form of aldose reductase exhibited monophasic kinetics with glucose and glyceraldehyde, whereas the unactivated or native enzyme exhibited a biphasic kinetics with both the substrates. The activated enzyme was less susceptible to inhibition by aldose reductase inhibitors such as sorbinil, alrestatin, and quercetrin as compared with the unactivated enzyme. Similarly, the native enzyme was strongly inhibited by some of the phosphorylated intermediates of glycolytic pathway, such as 3-phosphoglycerate, 1,3-diphosphoglycerate, 2,3-diphosphoglycerate, and ADP,whereas the activated enzyme was either not inhibited or inhibition was 20–30% only. Partially purified aldose reductase from the normal human lens exhibited properties similar to the native enzyme of other tissues, whereas the enzyme from clear lens obtained from diabetic subjects with severe hyperglycemia expressed properties similar to the in vitro activated enzyme of aorta, brain, and muscle.