The dye dilution technique can provide useful information regarding the newborn cardiovascular system if attention is paid to certain limitations imposed by the peculiarities of the age group being studied. It is necessary that an accurate dye injection system be used and that analysis of the dye curves be done in specified manners. All the newborns studied had a left-to-right shunt present during the first 2 hours of life. The size of the shunt was relatively constant, averaging 35%, and was not influenced by route of delivery. Although not proven by our studies, it seems probable that this shunt is through the ductus arteriosus. A right-to-left shunt, also presumed to be at the ductal level, was present in all babies up to the age of 40 minutes. After the age of 60 minutes, the majority no longer demonstrate such a shunt. The average left ventricular output for the entire group was 3.9 1/min/m2. Left ventricular output and right ventricular output were both significantly greater in babies delivered vaginally than in those delivered by cesarean section, the difference being most marked at age 60 minutes. Since the size of the left-to-right shunt was the same for both groups, pulmonary blood flow was also higher in vaginally delivered babies. The significance of these important physiologic findings in terms of neonatal pathology, has not been determined.