Relationships between plasma angiotensin-converting enzyme (ACE) and cortisol, and blood pressure were examined in chronically catheterized ewes and their fetuses during late gestation (111-141 days, term 145 +/- 2 days). Plasma ACE was also measured in non-pregnant adult ewes and in lambs over the first 5 weeks of life. In fetuses near term (136-141 days), plasma ACE was greater than in those studied earlier in gestation; overall, plasma ACE correlated with gestational age (r = 0.72). The ontogenic rise in plasma ACE was associated with prepartum increases in plasma cortisol (r = 0.67) and blood pressure (r = 0.66). No relationship was observed between plasma ACE and partial pressure of oxygen in utero. Peak plasma ACE concentration observed in fetuses near term was maintained in newborn lambs for 3 days after birth. By 2 weeks of postnatal age, plasma ACE had decreased to the value seen in non-pregnant adult ewes. Maternal plasma ACE was similar at all gestational ages studied, and was lower than that observed in non-pregnant ewes. Therefore, in the sheep fetus, plasma ACE increased towards term in association with the prepartum cortisol surge. Developmental changes in ACE activity may be partly responsible for the ontogenic rise in fetal blood pressure.