In rats the "fast" fibular nerve was transposed to the "slow" soleus muscle outside the original innervation band. Formation of new neuromuscular junctions was induced by cutting the soleus nerve after different periods of time. The morphological maturation of these junctions was studied by electron microscopy. New neuromuscular junctions do not form when the original innervation is left intact. Three to five days after denervation, vesicle-laden terminal boutons contact muscle fibers with only the basal lamina of the latter intervening. Three weeks after denervation, most boutons are larger and postsynaptic folds are present, although younger stages are also seen. Sixteen weeks after denervation, the neuromuscular junctions appear mature. This corresponds well with electrophysiological findings in the same material. The fully developed neuromuscular junctions sixteen weeks after denervation possess postsynaptic folds similar to those of normal "fast" muscle fibers. This suggests that the "fast" fibular nerve rather than the "slow" soleus muscle fibers determines the morphology of the postsynaptic folds. Possible trophic neuromuscular interactions are discussed.