Hemodynamic observations were made in 71 normal newborn infants up to 60 hours of age. A right-to-left or bidirectional shunt through the ductus arteriosus was demonstrated in 8 of 63 infants. Evidence of a definite left-to-right shunt was found in 7 of 30 infants. The data indicate that a bidirectional shunt is present shortly after birth and may persist for as long as 6 hours. This eventually becomes entirely left-to-right, and by 15 hours of age the shunting of blood either becomes physiologically insignificant or ceases entirely.