Serum angiotensin-converting enzyme (SACE) levels were determined in normotensive pregnant subjects and patients with pre-eclampsia in the third trimester of pregnancy and 5 days, 3 and 6 months after parturition, and also in the first and second trimester in the normotensive group. SACE levels were reduced during pregnancy and 5 days after delivery in the normotensive subjects compared with the levels 3 and 6 months after delivery and to the non-pregnant control subjects. After correction for plasma volume expansion, SACE was reduced in first and second trimester only. In pre-eclampsia both SACE and corrected SACE were significantly lower in third trimester and 5 days after parturition than 3 and 6 months after delivery and in non-pregnant control subjects. The reason for the reduced level of SACE in first and second trimester during normal pregnancy is not known. The low SACE levels in pre-eclampsia may be a secondary phenomenon due to a decreased placental synthesis without primary importance for the blood pressure regulation.