Angiontensin-converting enzyme (ACE) catalyzes rapid conversion of angiotensin II (AII). This enzyme has been identified in the vascular endothelium of nearly every tissue. Inasmuch as AII is the biologically active component of the renin-angiotensin system, and since age-related differences exist in the renin-angiotensin system, it was of interest to determine converting enzyme activity during development. ACE activity was quantified by measuring the optical density of hippuric acid liberated from hippuryl-L-histidyl-L-leucine (HHL) following incubation with the 20,000 X g supernatant of tissue homogenates. Pulmonary ACE activity of near-term fetal rats was not different than 1-day-old animals. Therafter, ACE activity increased during the first 6 wk postpartum in a biphasic manner. A similar age-dependent increase in converting enzyme activity was observed in rat kidney, mouse kidney, and mouse lung. Substrate affinity of all enzymes measured was similar, suggesting that the age-related increase in activity was due to increased enzyme content. The low activity of ACE in the newborn might function to limit AII production.