The present study was designed to quantitate the role of the renin-angiotensin system (RAS) in determining the chronic relationships between arterial pressure (AP), renal hemodynamics, and Na excretion. In six control dogs, Na balance was achieved during chronic step increases in Na intake from 5 to 500 meq/day with small increases in AP (<7 mmHg), moderate increases in GFR (19%), and decreases in filtration fraction (FF) and plasma renin activity. Similar increases in Na intake in six dogs with angiotensin II (AII) fixed, due to constant intravenous infusion of 5 ng . kg-1 . min-1 AII, caused large increases in AP (42%), GFR (31%) FF, and calculated renal Na reabsorption (TNa) above control. In six dogs with AII formation blocked with SQ 14,225, Na balance at intakes of 5-80 meq/day occurred at reduced AP, GFR, FF, and TNa, although plasma aldosterone concentration (PAC) was not substantially different from that in control dogs. At Na intakes above 240 meq/day, AP was not altered by SQ 14,225. These data indicate that during chronic changes in Na intake the RAS plays a major role, independent of changes in PAC, in allowing Na balance without large changes in GFR or AP. The mechanism whereby AII conserves Na chronically is through increased TNa, since steady-state TNa was increased by AII and decreased by SQ 14,225.