The active and passive components of NaCl absorption were examined in doubly perfused proximal convoluted tubules (PCT) of the rat kidney. When anion concentration gradients were generated by perfusing the lumen with a high chloride, low bicarbonate solution and the peritubular capillaries with a complete solution resembling plasma ultrafiltrate, volume absorption (JV) was 1.79 nl/mm/min and estimated chloride absorption (JCl) was 270 pEq/mm/min. When anion gradients were eliminated by perfusing the peritubular capillaries with a high chloride solution, JV was reduced to 0.91 nl/mm/min and JCl to 140 pEq/mm/min. These residual rates of absorption were reduced to zero by removing potassium from the perfusates. In the presence of anion gradients, removal of potassium reduced JV from 1.79 to 0.60 nl/mm/min and JCl from 270 to 90 pEq/mm/min. It is concluded that: (1) when PCT lumen are perfused with high chloride solution and the peritubular capillaries perfused with an ultrafiltrate-like solution, approximately 50% of NaCl and water absorption is passive, driven by the anion gradients, and 50% is active; (2) when PCT lumen and peritubular capillaries are both perfused with high chloride solution, anion gradients are absent and all NaCl absorption is active.