I n chronic sodium depletion the glomerular filtration rate may be reduced, and alterations in proximal tubular function may contribute to the maintenance of antinatriuresis. Measurements were made by micropuncture technique in superficial nephrons of the Munich-Wistar rat of (a) the determinants of glomerular filtration rate, (b) peritubular capillary hydrostatic and oncotic pressure, and (c) proximal tubular fractional and absolute reabsorption in both a control group (group 1, n = 12) and a group of chronically sodium-depleted rats (group 2, n = 12). Single nephron filtration rate (sngfr) was 37.2±1.2 in group 1 and 31.6±1.0 nl/min/g kidney wt (P < 0.05) in group 2. Of the factors potentially responsible for the observed reduction in sngfr, there was no change in systemic oncotic pressure or the transglomerular hydrostatic pressure gradient. Sngfr was lower in group 2 because of both a reduced single nephron plasma flow (rpf) (128±6 vs. 112±5 nl/min per g kidney wt, P < 0.05) and additionally to a decrease in the glomerular permeability coefficient, L_pA, from a minimum value of 0.105±0.012 in group 1 to 0.054±0.01 nl/s per g kidney wt per mm Hg (P < 0.01) after chronic sodium depletion. There was no difference in fractional proximal tubular reabsorption between group 1 and group 2. Absolute proximal reabsorption (APR) was reduced from 20.8±1.3 in group 1 to 16.3±0.9 nl/min per g kidney wt in group 2.The role of angiotensin II (AII) in maintaining glomerular and proximal tubular adaptations to chronic sodium depletion was assessed in subsets of groups 1 and 2 by the infusion of the AII antagonist Saralasin at a rate of 1 μg/kg per min. In group 1 rats, Saralasin had no effect on sngfr, rpf, or L_pA, because animals remained at filtration pressure equilibrium. In group 2 rats, AII blockade was associated with an increase in sngfr from 31.6±1.0 to 37.1±1.7 nl/min per g kidney wt (P < 0.01). Rpf increased during Saralasin infusion solely as a result of a decrease in afferent arteriolar resistance from 21.7±2.3 to 15.2±2.3 10⁹ dyn-s-cm⁻⁵ (P < 0.01). Saralasin infusion did not affect the reduced L_pA in group 2, as L_pA remained 0.056±0.02 nl/s per g kidney wt per mm Hg and rats remained disequilibrated. In spite of the increase in sngfr in group 2, AII antagonism further decreased APR to 13.1±1.5 (P < 0.01). Distal delivery therefore, increased from a control value of 15.3±1.3 to 24.3±1.5 nl/min per g kidney wt (P < 0.01).In conclusion, both a decrease in L_pA and a reduction in rpf were major factors mediating the decrease in glomerular filtration rate observed in chronic sodium depletion. Saralasin infusion revealed a significant effect of AII on rpf and afferent arteriolar resistance in chronic sodium depletion, but no effect of AII on either efferent arteriolar resistance or the decrease in L_pA could be demonstrated. Saralasin had no effect in rats that were not chronically sodium depleted. In group 2 rats AII antagonism reduced APR even though sngfr increased, suggesting an influence of AII on proximal reabsorption. The marked changes observed during Saralasin infusion in the chronically sodium-depleted rat reveal important modifying effects of endogenously generated AII on both the glomerulus and proximal tubule.