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The histological composition of the human rectus abdominis muscle was observed, using cross sections of the venter at navel level from 45 males, 45 nulliparous females and 45 parous females. Among males, nulliparous females and parous females in their twenties, muscle cytoplasm constitutes most of the total histological composition of cross sections of rectus abdominis muscle, while other tissue components are present in only extremely small quantities. There is, however, a distinct decrease in muscle cytoplasm with age, and an increase in connective tissue components and fat cells. Above the age of sixty the increase of fat cells is marked, and by the eighties fat cells constitute the largest percentage of all tissue components. Concerning the percentage of each element, muscle cytoplasm is highest among males, connective tissue components and fibrocyte nuclei highest among parous females. The percentage of fat cells is higher in females than in males. The total number of muscle fibers in cross sections shows a marked decrease with age in both sexes, males exhibiting a greater number than females, while parous females show the smallest number of muscle fibers. There is a discernable increase with age in the number of muscle fibers of large diameter.