Angiotensin-converting enzyme (ACE) catalyzes rapid hydrolytic cleavage of angiotensin I to form angiotensin II (AII). Inasmuch as converting enzyme activity is present at birth and increases postnatally to adult values it was of interest to determine the prenatal development of ACE. Converting enzyme activity was determined in the 20,000 x g supernatant fraction of lung homogenates using hippuryl-L-histidyl-L-leucine (HHL) as substrate. Hippuric acid liberated by the hydrolysis of HHL was quantified spectrophotometrically. ACE activity was first detectable at 18 days of gestation and increased fourfold prior to birth (21 days gestation). Pulmonary ACE activity of 1-day-old animals was twice that of fetuses at day 20 of gestation; however, this increase did not appear to result from ventilation alone. The Michaelis-Menten constant for fetal ACE (2.0 mM HHL) was not different from that calculated for ACE of adult rat lungs (2.6 mM). These data were interpreted to indicate that the age-related increase in ACE activity was due to greater ACE content as opposed to further activation of preexisting enzyme. This increase in fetal ACE activity may play an important role in preparing the renin-angiotensin system for postnatal function.