Comparative studies of mutarotase [aldose 1-epimerase, EC 5.1.3.3] from the kidney, liver and small intestine of rats were performed placing in the focus on the study of multiple forms. The findings obtained are as follows. Mutarotases from the kidney and liver of adult rats were both separated into four forms (types I-IV) by DEAE-cellulose column chromatography, whereas only two forms (types I and II) were detected in the small intestine. Liver mutarotase type I was further separated into types I1 and I2 by column chromatography on hydroxylapatite. Types I and II from the kidney and type II from the liver were purified to homogeneity as judged by isoelectric focusing on thin layer polyacrylamide gel. Of various physicochemical properties, only the Km for alpha-D-xylose and the isoelectric point were different among the multiple forms. Liver mutarotase was immunohistochemically localized in the nuclei of parenchymal cells and small intestine enzyme in the nuclei of mucosal cells, indicating similarity with the localization of kidney enzyme (in the nuclei of epithelial cells of renal tubules and glomeruli) which was reported in our previous paper [Experientia (1979) 35, 1094-1097]. The kidney mutarotase level increased gradually after birth and reached a maximum near adult level within 20 days. This developmental pattern was essentially the same as that in the liver but clearly different from that in the small intestine, in which the mutarotase activity of suckling rats was several times higher than that of adult rats. Distribution patterns of multiple forms (types I-IV) of the enzyme in the kidney and liver of 10-day-old rats were similar to those in respective tissues of adult rats. On the other hand, the small intestine of 10-day-old rats contained four forms (types I-IV), whereas there were only two forms (types I and II) in adult rats.