In 16 dialysis patients, 8 hypotensive and 8 normotensive, and 8 control subjects, blood volumes were studied and angiotensin II (AII), aldosterone (Aldo) and arginine vasopressin (AVP) were measured in the supine position, after standing upright for 5 min and after a standard exercise test. In 8 hypotensive and 7 normotensive dialysis patients and in 7 control subjects, the pressor response to AII infusion was determined. Baseline values for AII, Aldo and AVP were the same in the different groups but with a much wider range in uremic patients. After standing for 5 min. both hypotensive and normotensive dialysis patients were able to maintain blood pressure without a significantly greater increase in heart rate than was seen in controls. Contrary to controls, however, all non-nephrectomized dialysis patients responded with an increase in AII and Aldo after standing for 5 min. In controls, exercise induced a clearcut increase in AII, Aldo and AVP, while no such changes were detectable in either hypotensive or normotensive dialysis patients. No significant differences in blood volumes were detectable between the groups. The pressor response to AII infusion was decreased in both normotensive and hypotensive dialysis patients compared with controls. It can be concluded that circulatory adjustment is abnormal in both hypotensive and normotensive dialysis patients as indicated by extremely high baseline levels of AII in some patients, abnormal response of the renin-angiotensin-aldosterone system and AVP to postural change and exercise and a decreased pressor response to AII infusion. However, neither these abnormalities nor changes in blood volumes can directly account for the chronic hypotension seen in some dialysis patients.