Aldose reductase (EC 1.1.1.21) from human brain has been purified to apparent homogeneity. The enzyme catalyzes the NADPH-dependent reduction of several physiological and xenobiotic aldehydes. Isocorticosteroids, e.g. isocortisol and isocorticosterone, are the best substrates (Km less than 1 micron), followed by aromatic and arylalkyladehydes, including biogenic aldehydes (Km = 3 - 15 microM). The activity towards aldoses is highest with glyceraldehyde (Km = 25 microM) and decreases with increasing number of carbon atoms of the sugar. Flavonoids, e.g. quercetin and rutin, inhibit aldose reductase (IC50 = 2 - 5 microM). Sulfate ions, on the other hand, stimulate the enzyme activity. Thiol-modifying reagents, e.g. 4-hydroxymercuribenzoate and iodoacetate, cause a time-dependent inactivation. Aldose reductase consists of a single polypeptide chain with a molecular weight of 38 000 and an isoelectric point of 5.9. In the presence of thiol reagents the isoelectric point is shifted to 5.1. Antibodies against aldose reductase do not cross-react with other carbonyl reductases, Nevertheless, the comparison of structural and enzymic properties of aldose reductase with those of other carbonyl reductases suggests a relationship between aldose reductase and aldehyde reductase (EC 1.1.1.2).