The number of functional motor units in normal, acute myopathic, and reinnervated rat solei was determined by an electrophysiologic method. The number of motor units was about the same as half of the number of mye-linated fibers larger than 5μ in diameter (alpha fibers) in the nerve to the soleus. Normal and myopathic rat solei were found to have about 30 units; reinnervated solei had about half this number. Maximum isometric twitch and tetanic tensions of the reinnervated solei approximated the values in normal muscles, whereas these parameters were reduced by an average of 65 percent in myopathic muscles. The findings are in keeping with theoretical predictions about the fate of motor units in acute myopathic disease and most neurogenic diseases.