1. The renal response to a low-sodium diet alone and a low-sodium diet plus the daily oral administration of chlorothiazide was examined in rats. Sodium restriction resulted in a decrease in sodium excretion until day 4, after which it remained constant. The administration of chlorothiazide resulted in an initial natriuresis. By day 6, however, the natriuresis had abated and thereafter sodium excretion remained the same as that of the low sodium group. 2. After the animals were in balance on their respective regimens, clearance and micropuncture studies were performed. The glomerular filtration rate was lower in the chlorothiazide-treated rats than in control rats and/or in the low-sodium group. End proximal tubule TF/Pinulin ratios were higher in the diuretic-treated animals than in control rats. TF/Pinulin ratios in low-sodium animals were lower than in the diuretic-animals but higher than in control rats. 3. These studies demonstrate that the escape from the chronic effects of chlorothiazide is due to a decrease in the glomerular filtration rate and to an increase in fractional reabsorption in the proximal tubule, resulting in a reduction in delivery of filtrate to the cortical diluting segment where chlorothiazide exerts its major inhibitory effect.