1. Calculations from the Fick diffusion law are shown to predict that membrane equilibria should be established during the course of ultrafiltration.
2. It is shown that the chloride ion is more concentrated and the sodium ion less concentrated in the ultrafiltrate than in the plasma from which the ultrafiltrate was derived.
3. It has been found that by increasing the base bound by protein through a reduction in the bicarbonate content the difference between the plasma concentration and the ultrafiltrate concentration for the several ions studied increases.
4. Calculations from the Donnan equation as to the magnitude of the change in base bound by protein at differing hydrogen ion concentrations are in substantial agreement with the observed values, thus rendering it probable that the membrane equilibrium effect is responsible for the change in distribution ratios observed.
5. It is pointed out that the observed difference in the distribution ratio of cations from that of the chloride anion is probably to be explained by the influence of protein in lowering the activity coefficient of cations when on the alkaline side of the isoelectric point.
6. It is pointed out that account must be taken of these observations in any consideration of the rôle of ultrafiltration in the production of any secretion or body fluid.