In order to determine the degree of exchange diffusion between the tubular cells of the rat kidney and the tubular lumen, microinjections of labeled amino acids and inulin have been carried out in peritubular capillaries. The animals were undergoing mannitol diuresis. Fractionated urinary collections were made from the ureters of both kidneys after each microinjection in order to determine the content in radioactivity. L-Arginine-<sup>14</sup>C and inulin-<sup>3</sup>H or L -leucine-<sup>3</sup>H and inulin-<sup>14</sup>C were injected. Our data show that: (1) Inulin excretion follows a similar pattern in both the control and the experimental kidney. (2) The amino acid excretion is almost as large in the control kidney as in the experimental one. However, the quantity found in the experimental kidney is always greater, this supplementary fraction being 2.0 ± 0.44% of the injected dose. (3) In experiments carried out under a perfusion of unlabeled amino acids, the supplementary fraction excreted by the experimental kidney increases slightly to 3.9 ± 1.03%. Our results indicate that the renal tubular cells are permeable from the basal membrane towards the tubular lumen; however, the fluxes in this direction are not as large as suggested by previous in vitro studies.