The effects of muscular activity on the motor end-plates were studied. To simulate an overload training on the diaphragm, Japanese waltzer mice were subjected to hypobaric hypoxia (simulated altitude of 3000 m) for 7 and 14 days. After demonstration of the end-plates by AChE-staining, their size and total number in the sternal part of the diaphragm were measured at the light microscopic level. The end-plate regions were enlarged after 7 days and were increased in number after 14 days. These changes can probably be explained by the mechanisms of terminal and collateral sprouting. The electron-microscopic examination showed no ultrastructural peculiarities in either of the experimental groups. The muscle fibers of the trained animals showed a strong reaction of AChE; thus, it was suggested that end-plate proliferation was myogenically induced.