The effect of propranolol on water and sodium depletion provoked by frusemide was studied. Nine hypertensive patients were treated at 2-day intervals with a single dose of 40 mg of frusemide associated with a placebo or propranolol (320 mg given in 8 doses over a period of 24 hours). The association frusemide-propranolol is more hypotensive than frusemide alone. Propranolol reduced the rise in plasma renin activity and aldosteronuria provoked by the diuretic; total sodium output/24 h remained stable but sodium elimination was increased in the later samples. This prolongation of the natriuretic action is correlated with the inhibition of hyperaldosteronuria. Propranolol depresses urinary osmolality for a longer period than frusemide alone and water elimination tends to be higher. This action of the beta-blocker on urine concentration is independent of its effects on aldosteronuria but is correlated with the inhibition of the rise in plasma renin activity.