Angiotensin-I-converting enzyme (ACE) activity was measured by a fluorimetric technique in serum samples taken from rat fetuses and their mothers. The samples were collected from anesthetized rats on consecutive days from day 18 of gestation to day 22 (day of delivery) and one day after birth. Fetal ACE activities were significantly higher than the corresponding maternal values which remained nearly unchanged during pregnancy. No dramatic changes in serum ACE activity of the developing fetuses could be observed.