1. The effect of osmotic pressure on the nerve resting potential of frog sciatic nerve is in accordance with the assumption of a membrane potential; increased osmotic pressure raises, decreased osmotic pressure lowers the potential.
2. The potential of crab nerves is affected by organic and inorganic cations in the approximate series:
Rb > K = diamylamine > dibutylamine > guanidine > tetraethylamine > diethylamine = dimethylamine > dipropylamine > tetramethylamine = choline = Na = Li.
3. The response of the potential to the series of dialkylamines (first decrease, then increase of response ascending in the series) is best understood by the assumption that the nerve membrane is a porous structure.
4. With respect to these salts as well as to other organic cations the dried collodion membrane as a model of a porous membrane shows a striking parallelism to the nerve membrane.
5. Both inorganic and organic anions (NO3, SCN, acetate, propionate, butyrate, lactate, pyruvate) have a definite, if slight, effect in raising the potential of crab nerves. This effect of anions indicates that the nerve membrane is not completely anion impermeable.
6. The effect of organic ions is, with certain restrictions, reversible. Its possible relation to the resting potential and to the after potentials of the electrical disturbance is discussed.
7. The response of the myelinated sciatic nerve of the frog and of the non-myelinated nerve of the spider crab show considerable agreement. There are some definite differences which are, however, not necessarily due to differences of the cell membranes involved, but may be ascribed to the difference of ionic conditions in Ringer and sea water.