Following neonatal nerve injury fast skeletal muscles recover less well than slow ones. This is because many muscle fibers are lost during reinnervation. Since fast muscles normally contain a small population of slow muscle fibers, we have used a monoclonal antibody to slow myosin heavy chains (SMHC) to study their number and pattern of distribution in fast muscles following temporary denervation at 5-6 days of age and subsequent reinnervation. During this time the original distribution of slow fibers changed to one showing irregular grouping, indicating that reinnervation of muscles after neonatal nerve injury is as nonselective as it is after nerve injury in adults. Despite a large reduction in the total number of muscle fibers during reinnervation, the number of slow fibers did not decrease. Thus muscle fiber loss was at the expense of the fast motor units alone.