The 24-h urinary excretion of prostaglandins (PG) E2 and F2 alpha, which reflects the renal synthesis of these substances, was measured by radioimmunoassay in 10 patients with essential hypertension before and after treatment with propranolol. In addition, changes in the urinary excretion of PGE2 and PGF2 alpha after furosemide injection were also studied before and after propranolol treatment. The urinary excretion of PGE2 was significantly increased after propranolol treatment, whereas that of PGF2 alpha did not change. An enhanced response in PGE2 excretion after furosemide injection was observed after propranolol treatment. In contrast PGF2 alpha increased in response to furosemide before, but not after, propranolol treatment. Changes in renal hemodynamics induced by propranolol seem to be the main factor in PGE2 alpha increase. However, an indirect effect of beta-blockade or a direct effect on the enzymes governing renal PG synthesis cannot be ruled out.