To assess directly the effect of increasing and decreasing peritubular protein concentration on active NaCl absorption, rabbit proximal convoluted tubules were perfused with a high-chloride solution simulating late proximal tubular fluid and bathed in a similar solution containing 0, 6, or 10 g/dl albumin. A direct examination of the active component of NaCl absorption was ensured by 1) the reduction of luminal solutes other than NaCl (no glucose or alanine and only 5 mM HCO-3), and 2) the use of perfusate and bathing solutions in approximate Donnan equilibrium, so that electrochemical driving forces responsible for passive net NaCl absorption are eliminated. When bathing solution albumin concentration was reduced from 6 to 0 g/dl, volume absorption (Jv) was reduced significantly from 0.39 +/- 0.07 to 0.22 +/- 0.08 nl X mm-1 X min-1. When bathing solution albumin concentration was increased from 6 to 10 g/dl, Jv increased significantly from 0.26 +/- 0.04 to 0.37 +/- 0.04 nl X mm-1 X min-1. When proximal convoluted tubules were bathed in 0, 6, or 10 g/dl albumin at 20 degrees C, Jv was not different from zero in all periods, confirming that changes in bathing solution albumin require the presence of active transport to influence NaCl absorption. The transepithelial potential difference attributable to active transport was not different from zero in all periods of all protocols, demonstrating that active NaCl absorption was electroneutral. In conclusion, these data show directly that peritubular protein concentration modulates neutral active NaCl absorption in the rabbit proximal convoluted tubule.