The aim of the present investigation was to determine, by quantitative electron microscopy, the effects of a 5-wk tail-suspension period on rat soleus muscle ultrastructure. A marked decline (-60%) in muscle mass occurred. The mean fiber cross-sectional area decreased to a greater extent (-75%) than the capillary-to-fiber ratio (-37%), leading to a higher capillary density (+148%) after hypokinesia. The total mitochondrial volume density remained unchanged, whereas the volume density of myofibrils was slightly but significantly reduced (-6%). A shift from subsarcolemmal to interfibrillar mitochondria occurred. Interfibrillar mitochondrial volume density was highest near the fiber border and decreased toward the fiber center. An increase in volume density of satellite cells suggested muscle regenerative events. Soleus atrophy with tail suspension greatly decreases the muscular volume but leaves the ultrastructural composition of muscle fibers relatively unaffected.