In examininig the influence of alterations in the inter-polar length of galvanic stimulation on the excitability of a frog's sciatic-gastrocnemius organisation, many experiments were made in which direct current and induction shock and several different arrangements for stimulation were employed. In all this apparatus for electrical stimulation, special precautions were taken to obviate alteration of the current applied. There are three principal theories current on the question of the effect of altering the inter-polar length of galvanic stimulation on nerve excitability. The first is that the strength of the stimulating current required to produce the minimum muscular twitch becomes lower as the inter-polar length increases. The second, on the contrary, maintains that an opposite effect is achieved; that is to say, that the strength of the current decreases when the inter-polar length increases, because the electrical resistance is thereby also increased. The third opinion is that the effect is in inverse relation to the ascending or descending direction of the current. Reviewing these theories in the light of the results gained from the present experiments, the following conclusions are submitted. (1) The excitability of the frog's sciatic nerve, at first rapidly and then gradually, increases as the interpolar length of the stimulating current is increased. (2) The results of Marcuse and Tschirjew, who insist that the increase of nervous excitability reaches its maximum value at a definite inter-polar length, were not sustained by these experiments. (3) The effect of alteration of the inter-polar length on nerve excitability is independent of the ascending or descending direction of the current, i.e., the direction of the current has no effect on the degree of excitability. (4) The chronaxie of a frog's sciatic nerve becomes shorter as the inter-polar length increases. (5) The phenomenon of polarisation in the nerve fibre, caused by electrical stimulation increases directly with the inter-polar length, because of the reversed direction of ionic diffusion. (6) The influence of a magnetic field, brought to bear on the nerve fibre reduces its excitability.