The mechanism by which the osmotic pressure increases in tubular fluid along the descending limb of the loop of Henle was examined in Psammomys undergoing salt diuresis. In two series of experiments, micropuncture samples were collected either from proximal and distal convolutions at the surface of the cortex, or from loops of Henle and collecting ducts at the surface of the extrarenal part of the papilla. Inulin-3H, urea-14C, Na+, and K+ concentrations, as well as osmotic pressure, were determined in all micropuncture samples.
