Micromeasuring studies have been made on the senile changes of the surgically resected minor pectoral muscles in 200 Japanese females of various ages (26-80 years). The microscopic differentiation between type I (red) and type II (white) fibers were made by the histochemically demonstrated myosin-ATPase activity with frozen sections and also by the immunohistochemical demonstration of beta-enolase in the paraffin sections with indirect antibody method. The decrease with age in weight of the muscle was not markedly observed. Muscle fibers of type I were significantly decreased in number and increased in size after 60 years of age. However, size of muscle fibers of type II decreased gradually after 40 years of age. However, size of muscle fibers of type II decreased gradually after 40 years of age. Total volume of type I muscle fibers was not changed according to age, on the contrary, that of the type II fibers decreased significantly after 60 years of age.