Plasma concentrations of angiotensin II (AII), aldosterone, 18-hydroxycorticosterone and cortisol were measured in seven patients with benign essential hypertension and in seven age-matched control subjects before, and at frequent intervals for 2 h after the intravenous injection of frusemide (40 mg). In the normal subjects, significant increases in the plasma levels of AII, aldosterone and 18-hydroxycorticosterone were apparent from 15 min after diuretic administration. The integrated responses of each hormone to frusemide administration were calculated. Aldosterone and AII responses to the diuretic were closely related, although three hypertensive patients had normal integrated aldosterone responses despite subnormal increases in the plasma concentrations of AII. The integrated 18-hydroxycorticosterone responses were greater in the hypertensive (median 970 nmol.h h-(1).1-(1)) than in the normal subjects (median 180 nmol.h-(1).1-(1)), P<0.05. Some patients with a raised blood pressure appear to have an enhanced adrenal corticosteroid response to frusemide; this probably reflects an increased sensitivity to angiotensin II.