Plasma concentrations of arginine vasopressin (AVP), angiotensin II (AII), aldosterone (Aldo), urinary output (V), osmolar clearance (Cosm), free water clearance (CH2O), fractional excretions of sodium (FENa) and potassium (FEK), urinary sodium excretion rate (U-Na) and serum potassium (S-K) were determined in 9 patients with essential hypertension (group I) and 13 normotensive healthy control subjects (group II) before and three times during the first 4 hours after an intravenous injection of 40 mg of furosemide. AVP, AII, Aldo, V, Cosm, FENa, FEK and U-Na increased in both groups. However, the elevation in AVP was significantly more pronounced and the rate of increase in Aldo was significantly slower in group I than in group II. There were no significant differences in AII, V, Cosm, CH2O, FENa, FEK and U-Na between the groups. S-K was significantly reduced only in group I. AVP and AII were not significantly correlated to each other or to blood pressure. It is suggested that the responsiveness of the renal tubules to AVP is reduced in essential hypertension and that the larger increase in AVP might be a compensatory phenomenon. The slower increase in Aldo in essential hypertension could be attributed to the reduction in S-K.