THE DIFFERENTIAL response to tenotomy of the fast (pale) and the slow (red) muscles of the rabbit and cat has been examined by several workers. In comparison to fast muscles, the tenotomized soleus (slow muscle) atrophies rapidly1-3and during this process it becomes increasingly sensitive to depolarizing agents,4shortens its contraction time,5and shows striking morphological and histochemical changes by light microscopy.6In the present study, we report on the electron microscopic aspects of the structural changes in the tenotomized soleus. Materials and Methods Rats weighing about 200 to 400 gm were anesthesized with ether, the common Achilles tendon was completely severed from its attachment, and a segment 2 mm in length was excised. Ten rats were used and they were sacrificed (two each time) at 1, 11/2, 2, 3, and 4 weeks after tenotomy. Specimens from tenotomized and control soleus muscles were held at rest