Plasma concentrations of arginine vasopressin (AVP), angiotensin II (A II), aldosterone (Aldo), serum osmolality (Sosm), urine volume (V), and free water clearance (CH2O) were determined in the third trimester of pregnancy, and 5 days and 3 months after delivery in pre-eclampsia (group I), in normotensive pregnancy (group II), and in non-pregnant control subjects (group III). The AVP was the same in the third trimester of pregnancy in groups I and II and did not deviate significantly from the level in group III. However, 5 days after delivery, AVP was lower and V and CH2O higher in group I than in group II. There was no correlation between AVP and Sosm in the third trimester in either group I or II, but 5 days after delivery a significant positive correlation was found between these parameters in both groups I and II as well as in group III. The A II and Aldo changed qualitatively in the same way in groups I and II, that is, considerable elevation in the third trimester and normalization after delivery. Also, A II and Aldo were lower in group I than in II. The AVP and A II were not correlated and there was no significant relationship between systolic or diastolic blood pressure on the one hand and AVP, A II or Aldo on the other in either group I or II. Thus the osmoregulatory system appears to be altered in both pre-eclampsia and normotensive pregnancy, but becomes normal again 5 days after delivery. In pre-eclampsia a suppression of AVP seems to be responsible for the elimination of excess water in pre-eclampsia 5 days after delivery. There was no evidence for a causal relationship between blood pressure and the osmoregulatory system or the renin-angiotensin-aldosterone system in any of the pregnant groups.