1. Two series of experiments were carried out to determine the permeability of the renal cortical peritubular capillaries to albumin under control conditions in the concentrating kidney and after an infusion of hypo-osmotic fluid amounting to 6% body weight. 2. In the first series of these experiments the turnover of the interstitial albumin pool of the renal cortex was studied. Specifically, the mean transit time of albumin molecules from arterial plasma to renal lymph was measured in nine rats of each group. Mean values of 39.0 +/- 2.8 and 30.6 +/- 2.2 (S.E. of mean) min were found for the control and infused groups, respectively. 3. In the second series of experiments the interstitial distribution volume of plasma albumin in renal cortex was determined in twenty-three control and twenty-two infused rats. Mean values of the extravascular distribution volumes were 1.66 +/- 0.21 and 1.37 +/- 0.18 (S.E. of mean) microliter/100 mg tissue, respectively. 4. The unidirectional clearance of albumin from the capillary to the interstitium in the control and infused groups, respectively, was calculated to be 7.1 +/- 1.0 and 7.5 +/- 0.9 (S.E. of mean) ml. sec-1 10(-4) in 100 g cortical tissue. 5. For the reabsorbing surface of the peritubular capillaries in the renal cortex, a lower bound was calculated for sigma, the reflexion coefficient of albumin. The reflexion coefficient was found to be higher than 0.998 under both experimental conditions.