Degeneration at the neuromuscular function following cutting the phrenic nerve at the 9th intercostal space differs in red, white and intermediate skeletal muscle fibers. The ultrastructure of the nerve terminal and the muscle fiber between 12 hours and 21 days following denervation suggests that lack of neurotrophic influences results in responses specific for each fiber type. Degeneration of axon ends is rapid and by 2 days axon terminals are missing from the end-plate areas of all 3 fiber types. Schwann cells "engulf" degenerating axon terminals and eventually replace them in the primary clefts. Schwann cells display specific morphological changes directly related to axonal degeneration. In all instances axon terminal degeneration precedes muscle fiber degeneration. Synaptic cleft changes are similar for all types of muscle fibers. Primary cleft structure appears to be dependent upon neurotrophic influence, whereas secondary cleft structure is relatively unaffected by denervation. Initial changes in subsynpatic regions of muscle fibers include focal loss of sarcomere alignment and skewing of the Z lines. By 21 days myofibrillar disorganization appears most severe in white fibers and least in red muscle fibers. The rate and degree of degeneration of the axon terminal and subjacent muscle fiber are different for each of the 3 muscle fiber types.