Morphometric changes produced by repeated stimulation of the neuromuscular junctions (NMJs) have been studied by electron microscopy in 769 profiles of normal, heterozygous dystrophic, and dystrophic mice. The study indicates that in animals not stimulated but anesthetized for 40 min, there is no significant difference in the profile area between the normal, the heterozygous dystrophic, and dystrophic mice. There is also no significant difference between these three groups in the number of clear vesicles and of coated vesicles per micron2 of terminal. Following stimulation, the profile area of NMJs is not significantly increased in the normal and in the heterozygous dystrophic mice. However, the profile area is increased by 50% in the dystrophic animals. This is perhaps due to changes in the shape of the nerve terminal in stimulated dystrophic animals. However, the number of the clear vesicles per micron2 is significantly reduced to a similar extent in the three groups. These results indicate that the recycling of vesicular membrane is normal in dystrophic animals.