The circulating blood volume changes, plasma electrolytes, aldosterone as well as plasma-renin-activity (PRA) were studied in 14 patients following open-heart surgery with cardiopulmonary bypass (CPB) and in 14 patients following cardiac surgery without CPB. In both the groups a postoperative decrease of circulating blood volume was observed. This decrease occurred in spite of the positive fluid balance, and so a shift of fluid from vascular to extravascular compartment was likely. In the bypass group the plasma aldoserone level rose immediately after the operation and reached the peak on the first postoperative day. In the non-bypass group the aldosterone level started to fall after the operation, and reached the lowest level on the third postoperative day. The pattern of the postoperative PRA changes was almost identical with the aldosterone level changes in both the groups studied, which suggests that the activation of renin-angiotensin-aldosterone axis might be responsible for the observed plasma aldosterone changes.