Electrophysiological techniques were used to evaluate volume changes of compartments interposed between the peritubular membrane of proximal tubue cells and capillary wall or peritoneal surface of Necturus kidney. The time courseof potassium-induced peritubular membrane depolarization was measured during a switch from 3- to 90-meq potassium sulfate solution in superfusion as well as in vascular perfusion experiments. When superfusion fluids contained 6, 2, and 0 g dextran/100 ml, mean half times of depolarization in normal kidneys were 4.1, 1.9, and 5.8 s. Decreasing the colloid osmotic pressure of superfusion fluids resulted in a significant increase in half-time values. Similar results were obtained in saline expansion. In controls the mean half time was 4.5s; in saline expansion it was 7.7 s. In vascular perfusions, mean half-time values were 225, 350, 355, and 450 s for dextran concentrations of 6, 4, 2, and 0 g/100 ml, respectively. These results indicate that the half time of depolarization increases significantly as the colloid osmotic pressure of perfusion fluids decreases. Estimates of effective unstirred-layer thickness on the peritubular side of proximal tubular epithelium indicate a decrease by about 25% when vascular colloid osmotic pressure is increased from 0 to 54 mmHg.