The ultrastructural organization of 40 soleus neuromuscular junctions from ten normal young adult male and female Sprague-Dawley (SD)-derived rats (Charles River Breeders, CD-Crl:COBS (SD)BR) has been studied. A smaller sample of motor endplates from the gastrocnemius, diaphragm, and extensor digitorum longus muscles of these rats as well as from the soleus muscles of two adult Wistar (W) rats (Crl:COBS(WI)BR) was included. Widespread ultrastructural reorganization was evident at the soleus neuromuscular junction during the growth period from three to five months of age. A major characteristic of reorganization is the presence of junctional folds not associated with axonal terminals; such sites occur within a single endplate adjacent to areas with typical intact synaptic associations. Additional features possibly related to remodelling are: 1) spatial separation of axonal terminals from the myofiber, 2) intervention of Schwann cell cytoplasm between an axon terminal and myofiber, 3) aggregates of satellite cells, and 4) folded or multilayered basal lamina. These features are most pronounced in the soleus muscle but occur to varying degrees in the neuromuscular junctions of other muscles of SD-derived rats. Distinctive characteristics of the rat soleus postjunctional sarcoplasm include the widespread occurrence of myofibrillar components, abundant free and membrane-associated polysomes, and triads oriented in various planes. Away from such discrete sites, myofibers possess the usual highly oriented organization of myofibrils, T tubules, sarcoplasmic reticulum, and mitochondria. The soleus muscle is a postural muscle that responds directly to rising workload imposed by continuous body growth during young adulthood by steady myofiber hypertrophy and conversion of motor units (Kugelberg, '76). This changing structural-functional relationship may be reflected also by ultrastructural remodelling of the neuromuscular junctions reported here.