To determine the duration of bidirectional flow through the ductus arteriosus and to confirm the time of functional closure of the ductus arteriosus, 25 normal full-term newborns were studied serially using Doppler color flow imaging beginning at 2-7 h after birth and continuing until no ductal flow was detected (defined as functional closure). At the first examination, blood flow was bidirectional in 19 of 25 infants and continuously left-to-right in the remaining six. Subsequent studies revealed that bidirectional ductal flow changed to continuous left-to-right flow in 17 of 19 infants, whereas two of 19 abolished the flow completely at the second examination. Three of the 19 infants still had bidirectional flow through the ductus arteriosus at 12, 18.5, and 24 h after birth. The percentage of newborns who had bidirectional flow through the ductus arteriosus decreased roughly as a negative exponential function of age. The earliest functional closure of the ductus arteriosus was at 8 h after birth: by 24 hours 44% and by 48 h, 88% were functionally closed. By 72 h of age, the ductus arteriosus was functionally closed in all 25 infants. This study shows that bidirectional flow through the ductus arteriosus changes to continuous left-to-right flow before the functional closure, but can persist up to 24 h after birth.