Measurements were made of the oxygen and carbon dioxide concentrations in successive samples of blood taken from the umbilical arteries in normal newborn infants. The oxygen content was found to rise considerably as early as the 3–4th min after birth. This points to a rapid pulmonary expansion and a changeover to gaseous exchange in normal infants performing normal initial respiratory movements. The normal range of blood oxygen concentrations within the first minute is from 6–18.8%. This shows that in normal births labor may terminate with various degrees of placental detachment; evidently when the oxygen content was 18%, labor terminated before detachment, while when it was only 6–8%, the infant was delivered after the placente hand separated to a greater extent.