Fatty acids of major phospholipids were evaluated in lung homogenate of rhesus monkeys. Studies were conducted at 5 fetal ages and in adult animals. The total concentrations of sphingomyelin, phosphotidyl ethanolamine and phosphatidyl choline increased progressively during fetal life, demonstrated at 2- to 3-fold increase during the 25-day period prior to full-term gestation, and remained at the same level or decreased in the lungs of adult animals. The predominant fatty acid esterifiedto sphingomyelin was stearic acid; that of phosphatidyl ethanolamine was oleic acid; and that of phosphotidyl choline was palmitic acid. These data and this species may be used to improve understanding of pulmonary development in the human fetus and the respiratory problems of prematurity.