1. Catecholamine plasma concentrations and urinary excretion were measured together with plasma renin activity in ten patients with essential hypertension and in five normal control subjects before and after a frusemide challenge.
2. The same procedure was repeated in the same subjects 3–4 days later after pretreatment with oxprenolol.
3. Noradrenaline plasma concentrations and urinary excretion increased significantly after frusemide in all cases, returning to normal values at 30 and 60 min. Adrenaline plasma concentrations and urinary excretion were unchanged.
4. Plasma renin activity increased significantly in seven patients with hypertension and normal renin basal values, remaining unchanged in three hypertensive patients with low-renin basal values.
5. Oxprenolol suppressed the response of noradrenaline and plasma renin activity to frusemide in all cases.