Eleven patients with essential hypertension were given hydralazine (25 mg b.i.d.) for 2 weeks, hydralazine and propranolol (80 mg b.i.d.) for the next 2 weeks, and propranolol alone for the last 2 weeks. The changes in the renal and systemic circulation elicited by the drugs were studied before start of medication and bi-weekly during the treatment, using non-invasive radioisotope techniques. Hydralazine alone did not alter mean arterial blood pressure (MAP), heart rate (HR), cardiac index (CI), effective renal plasma flow (ERPF), peripheral renin activity (PRA) and plasma aldosterone (Aldo) but when propranolol was added MAP fell 15.2%, HR 22.5% and CI 18.4%, while ERPF was unchanged. When hydralazine was withdrawn and propranolol was given alone, ERPF decreased 13.2%. Plasma aldosterone was unchanged, whereas PRA decreased during propranolol treatment. The reduction in ERPF elicited by propranolol, was highly significant (P less than 0.01). From the test sequence it appears that dihydralazine prevents this effect of propranolol on kidney function. These findings might have a direct bearing on the choice of antihypertensive treatment.