Male adult Wistar rats were treated for 8 weeks with an organosilicone [2,2-dimethyl-4-(chloromethyl)-1,3-dioxa-2-silacyclopentane] employing two different dosages (25 and 50 mg/kg body weight i.p. daily) and the changes in the tubular apparatus of the kidney were investigated employing histological and enzyme-histochemical methods. The effects, more pronounced at the higher dosage, were the following: The brush borders of the proximal tubules were stuck together and disintegrated; only few epithelial cells remained intact showing a decreased activity of nonspecific esterases and the increase of β-hydroxybutyric acid dehydrogenase. The nuclei of most of the epithelial cells in the distal tubules were dislocated towards the enlarged lumen and the cytoplasma showed a decrease of nonspecific esterases and an increase of β-hydroxybutyric acid dehydrogenase. The collagen fibers in the walls of the collecting tubules were dislocated and disintegrated with a discontinuous border of Mg2+-ATPase; the nuclei of the epithelial cells were pyknotic, the cytoplasma showed an increase of β-hydroxybutyric acid dehydrogenase. The induced changes were partially reversible after a recovery period of 8 wee