The relative composition of skin, muscle, and bone and their distribution patterns throughout the body are given for a series ofMacaca mulatta from 141 days conceptual age through adulthood. In terms of percent of total body weight, the musculature of these animals doubles during the first postnatal year whereas bone and skin decrease. Regionally, the muscles of the thighs, back extensors, truncal-forelimb and upper arms increase most markedly. The thighs double and the upper arms increase whereas the trunk, hands, feet, and tail decrease. The biomechanical implications of these changes for motor development are discussed.