APPLYING histochemical techniques devised by Friedenwald and Becker1, it was found2 that in normal male mouse liver (Fig. 1) and kidney, β-glucuronidase activity is mainly confined to the mitochondria. It was considered that the rise in glucuronidase activity shown by biochemical methods to occur in these organs when repair has commenced subsequent to damage3 might be reflected in a change in the cytochemical picture. This has been found to be the case in experiments with frozen sections of mouse liver regenerating after partial hepatectomy or carbon-tetrachloride poisoning, using 8-hydroxyquinoline glucuronide as substrate. In contrast to the mitochondrial site in normal liver, β-glucuronidase activity as detected by this method now appears to correspond in situation to the nucleus (Fig. 2). This did not result from aggregation of the mitochondria, which were seen to be normally situated in sequent sections stained with Iron Haematoxylin after fixation in Regaud's fluid. On the other hand, Feulgen staining in regenerating liver gave a similar picture to that obtained by the glucuronidase method. Normal liver sections stained by the Feulgen method did not in any way resemble those obtained histochemically.