The number and the size of different populations of nuclei were studied in skeletal muscles and in the diaphragm of male Wistar rats of 200-250 g weight. Nuclei on cross-sections were counted and classified by electron microscopy, their incidence was corrected for their different lengths, and the number of nuclei per mm3 of muscle was determined by light microscopy. The total number of nuclei per mm3 was 5 · 104 in the superficial part of the anterior tibial muscle, it was 10 · 104 in the soleus, and it was 15 · 104 in the diaphragm. Half of the nuclei were localized inside muscle fibres. The incidence of satellite cell nuclei on cross sections was 4% of muscle nuclei in the anterior tibial muscle, and 8% in soleus and diaphragm. The number of satellite cells per mm3 muscle were 900, 4,900, and 5,300 in these muscles.
More than half of the satellite cells were closely associated with a capillary. In the anterior tibial muscle, and in 1-μm sections no satellite cells could be identified by light microscopy. In the soleus muscle and in the diaphragm, satellite cells were more rich in cytoplasm and many were visible in the light microscope.