Cr31labeled red cells and I131human serum albumin were injected intravenously into 8 mongrel dogs. After 1 hour kidneys were removed and frozen rapidly with dry ice. Sections of outer cortex, inner cortex, outer medulla, inner medulla, outer papilla and inner papilla were removed and analyzed for Cr51and I131radioactivity. Tissue radioactivity was compared with that of arterial blood and concentrations of labeled red cells and albumin were calculated. No significant difference was found in relative red cell and albumin content between outer and inner cortex. The magnitude of the I131albumin content in the cortex is interpreted to indicate a sizeableextnivascularexchangeable albumin pool. The renal papillae were extraordinarily deficient in red cells, but contained per 100 Gm., albumin equivalent to that of 39 ml. of plasma.