Cord blood angiotensin-I-converting enzyme (ACE) activity was examined in 21 term and 21 premature infants. Values were significantly higher in premature infants than in term infants. Cord blood ACE activity was found to have a significant negative correlation with birth weight and gestational age. ACE activity measured from peripheral blood during the first 24 h of life was higher in premature than term infants but similar to levels in maternal and adult controls. It was not possible to predict those premature infants who would develop respiratory distress syndrome on the basis of cord blood ACE activity. However, ACE may serve as a marker for maturation of the pulmonary vascular endothelial cell.