The morphology of satellite cells was investigated in skeletal muscle from mice of various ages between 7 days and 50 weeks. Satellite cells of very young muscle had abundant cytoplasm which was rich in organelles. Free ribosomes were abundant and usually arranged into polysomes of 5-6 units. Cisternae of rough endoplasmic reticulum were heavily studded with ribosomes and occupied the polar regions of the cytoplasm. Marked dilations of the cisternae, filled with an amorphous electron-lucent material, were a frequent and characteristic feature of satellite cells of very young muscle. The cytoplasm of young cells also contained a well developed Golgi apparatus as well as numerous mitochondria, microfilaments and microtubules. With increasing age there was a rapid reduction in organelles both qualitatively and quantitatively. For rxample, as the number of ribosomes decreased, their organization into polysomes was lost. The rough endoplasmic reticulum was present in cells of older muscle merely as small isolated profiles that lacked dilations. These and other features demonstrated during this study are consistent with the concept that satellite cells are metabolically very active in young muscle but rapidly become quiescent as the animal grows older.