result was not always obtained. From this we can easily estimate the refractory period of the nerve, which is present after reaching a state of equilibrium under certain conditions. If we assume ten stimuli per second to be the number required to produce slight fatigue when stimulation is prolonged, we can conclude that the refractory period in this state is somewhat longer than one tenth of a second. Even though _Gotch_ in his investigations already cited placed the refractory period of the normal nerve at about .005 second, this statement is in no way contradictory to the figure which we have just given. _Gotch_ measured simply the duration of the absolute refractory period of the normal nerve, in other words, the duration of the period in which no excitation at all could be brought about. On the contrary, my estimate, based upon the investigations of _Thörner_, refers to the _total_ refractory period of the nerve, that is, to the point of _complete_ recovery of the equilibrium of metabolism and of the specific irritability. Experimental proof of this assumption is already under way. I have endeavored to show the elementary principles at the basis of these extremely varied interference effects and to make a few generalizations concerning the complicated conditions here concerned. It has been shown that a great number of interference effects possess characteristics in common if one takes into consideration the process occurring in the course of a single excitation. The altered state which exists in living substance until the complete disappearance of excitation is the basis upon which to explain the altered effects produced by a second stimulus. This state alters during the whole course of the first stimulus until the original equilibrium of the metabolism of rest is, by self-regulation, again reached. It is, therefore, self-evident that the second stimulus must have different effects depending upon the momentary state of the living system at the time of its application. The state of the system differs depending on the length of the interval in which the second stimulation follows the first. The most important factor is the phase of the excitation period and the reduction of irritability. The second important factor is the intensity of the second stimulus; the relation of the two with each other determines the response. But the specific properties of the given systems must also be taken into consideration. It is important to know if the living system possesses isobolic properties, that is, every intensity of stimulation produces a _maximal_ liberation of energy, or if it possesses a heterobolic character, that is, stimuli of different strength bring about the liberation of _different_ amounts of energy. It is further important to know the rapidity of reaction, whether the system rapidly or slowly fatigues. In all cases it depends whether the second stimulus produces a perceptible excitation or whether it occurs