M. quadriceps and diaphragm were studied in Wistar rats 2, 3, und 4 weeks after 5/6 nephrectomy. Control animals were sham operated and pair fed. In 10 mu transverse cryostat sections, the following histochemical reactions were performed: NADH-dehydrogenase, myofibrillar ATP-ase, modified trichrom stain, hematoxylin-eosin. Three fibre types (I, II, intermediate) were analysed quantitatively by planimetry. Sarcolemnal nuclei per unit area and fibre cross section area were determined. In muscle specimens from uremic animals, a uniform atrophy of all three fibre types could be demonstrated. In contrast to findings in man, preferential atrophy of one of the three fibre types, increase of sarcolemnal nuclei per fibre cross section area or structural abnormalities within single muscle fibres could not be detected. Neurogenic damage could be excluded (electrophysiology, sciatic nerve planimetry). The fibre changes point to a primary disturbance of muscle metabolism.