Low dose iv infusion (0.01 and 0.03 micrograms/kg per min, for 30 min each) of alpha-human atrial natriuretic factor (alpha-hANF) produced a significant increase (+300%) in urinary protein excretion in patients with essential hypertension but not in normotensive controls, when their renal function was normal. The major component of excreted proteins induced by alpha-hANF infusion was presumed to be albumin on the basis of molecular weight (69,000) analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. Urine output and sodium and potassium excretion rates were increased dose-dependently by alpha-hANF infusion in the hypertensive patients in a similar fashion to those in the controls. Glomerular filtration rate (GFR) remained unchanged in the controls but was slightly increased in the patients (+33%) during the infusion. These results suggest that besides its previously recognized physiological functions such as natriuresis and diuresis, ANF plays an important role in the regulation of renal handling of proteins in patients with essential hypertension.