Since renal mechanisms by which ANP induces a natriuresis remain unclear, the effects of the infusion of 0.03 micrograms/min/kg body weight of rANP 1-28 into the right renal artery (via a very thin catheter inserted into the suprarenal artery) were studied in anesthetized rats using lithium clearance to monitor proximal tubular function. Intrarenal ANP did not cause changes in arterial pressure and hematocrit, nor in contralateral kidney handling of sodium. Its natriuretic effect on the right kidney resulted from an increase in the filtered load of sodium and from a decrease in reabsorption in the distal segments of the nephron. Proximal tubule sodium reabsorption increased in proportion to the filtered load.