By the third effect of electrodes of a polarizing current applied on a nerve is meant, as proposed by Suzuki and his coworkers (1, 2), the effect produced by a current of long or strong polarization and manifested by the changes in the nerve excitability under the electrodes in directions opposite to those of electrotonus.The changes in the excitability by electrotonus are due to the polarization potential itself, while those of the third effect result from certain physicochemical changes in the nerve membrane so that its ion-permeability is increased and the polarizability decreased under the cathode, while under the anode this relationship is reversed (Ebbecks (3, 4), Hecht (5), Suzuki (6)). In our laboratory the third effect has been examined with special reference to the threshold of stimulation as well as other values of nerve and muscle (Suzuki (7,8,9)).The changes in the excitability of nerve due to the third effect, as measured by the threshold as its index, can be observed by the "interruption method" (2), in which the polarizing current is interrupted just while the stimulation is applied. However, in the "superposition method", in which stimulations occur without opening the current, the changes due to the third effect is masked and distorted, especially in the anode, by the presence of polarization potentials, and the electrotonus-like changes are observed.The cause of this distortion can also be explained by the third effect theory (9).It has long been known that the nerve excitability shows momentarily, upon opening the current, the changes in directions opposed to those of electrotonus; these are named postcathodal depression and postanodal exaltation (Eccles (10)). These changes are attributed to the electrotonic effect of the counter current of depolarization, flowing through the electrode as a result of the cessation of polarizaton.Soeda (11) and Kikuti et al.(12) have measured the course and duration of these effects and demonstrated them to be of the order of msec.Since the third effect of polarization and the effect of the counter current are both of the same direction in excitability changes as to the signs of the electrodes, one could suspect that those observed by the interruption method are due to the counter current effect, though the latter is of very short duration Received for publication October 26, 1958.