Histochemical and electron microscopic examinations on biopsied quadriceps muscle in two cases with Down's syndrome (1-year-old and 6-year old girls): revealed the following findings. Histologically; the fundamental structure of muscle was well preserved; there were ob served no degenerated nor regenerated fibers but variability of moderate degree in fiber size was noticed. Histochemically type II fibers were smaller in size than type I, moreover, there was the type I fiber predominance, which was more prominent in the older case. Though the enzyme activities of muscle fibers were within normal limits in phosphorylase, succinic dehydrogenase, aldolase and myosin ATPase. Electro n microscopic examination showed no remarkable changes, except moderate disarrangement of the myofilament and increased myofibrillar glycogen. From our experience and literatures, the muscles o f most diseases of the central nervous system show type II fiber atrophy, but none of them shows the type I fiber predominance. So the type 1 fiber predominance found in our cases is considered t o be a specific histochemical feature of the muscle of Down's syndrome, suggesting another factor being involved in muscle pathology of Down's syndrome different from the other central nervous system diseases.
