Water and electrolyte excretion were studied during osmotic diuresis induced by infusion of concd, mannitol soln. into dogs. With increasing diuresis, relatively more water than Na was excreted in the urine. It is concluded that water and Na are not necessarily reabsorbed in a fixed ratio, and that the simultaneous reabsorption of large quantities of water is not essential to the reabsorption of Na; and that the reabsorption of the bulk of the filtered Na requires the expenditure of energy by the tubular cells as an active transfer process. Comparison of the concn. of osmotically active solutes in plasma and urine at high urine flows induced by osmotic diuresis shows that the reabsorbate is approx. isosmotic. It is concluded that associated with the active reabsorption in the proximal system of approx. 85% of the filtered Na, an osmotically equivalent quantity of water is reabsorbed by passive diffusion. The plasma-urine Na concn. difference tends to attain a constant value at creatinine U/P ratios below 3.5, indicating that this concn. difference is a major factor in limiting the proximal reabsorption of Na.