Blood volume (BV), extracellular volume (ECV), blood pressure (BP), creatinine clearance (CCr), plasma levels of angiotensin II (AII), aldosterone (Aldo) and arginine vasopressin (AVP), and serum osmolality (Sosm) were determined in 18 patients with adult polycystic kidney disease, 8 normotensive (group I), 10 hypertensive (group II), and in 11 control subjects (group III). ECV but not BV was increased in group I compared with group III, whereas BV and ECV did not differ significantly between groups II and III. In group II, Aldo and AVP were increased and AII tended to be increased, while in group I the hormone levels did not differ significantly from those in group III. Sosm did not differ significantly between the groups. In the combined patient group, CCr correlated positively with BV and ECV and negatively with BP. In the patients, AII and AVP were positively correlated with BP but not with CCr. The results suggest that both the renin-angiotensin system and AVP might be involved in the BP elevation, whereas expansion of ECV can be found without an increase in BP.