The natural sciences, if they are to retain their reputation for exactness and precision, require the strictest and clearest definitions of all conceptions. If we seek to penetrate more deeply into the varied happenings in concrete conditions, we must reconcile ourselves to dry pedantic definitions. In the case of that of the equilibrium of metabolism indeed we have before us one of the most important conceptions in physiology. The justification to speak of an equilibrium of metabolism arises from investigations of metabolism in mammals. The classical experiments of the previous century, as is well known, have shown that in the adult mammal receiving a necessary quantity of nourishment and in a state of rest, the intake and outgo of the constituent elements are the same. The carbon, hydrogen, nitrogen, oxygen, sulphur, phosphorus, etc., taken in during a lengthened period in the form of food and respired air, appear again in equal quantity, in other combinations, in the products of excretion of the organisms. Calorimetric experiments likewise show an equilibrium of the consumption and elimination of energy. If there thus exists an equilibrium of metabolism for the whole cell community, it is clear that the same must also apply to the individual cell, that is, for all living substance. The quantitative relations of the foodstuffs taken _in_, and the excreted metabolic products given _off_, are, however, merely a standard of the metabolism. We know that the former are used to build up new living substance and that the latter represent the result of disintegration of that previously existing living substance; for we find, as in the case of the plant, complicated protein combinations, which are built up from comparatively simple constituents of the food and are again broken down into comparatively simple substances. And so the building up and breaking down processes form the two great processes of metabolism, which with _Hering_[41] we can briefly call “_assimilation_” and “_dissimilation_.” In the terms assimilation and dissimilation are comprised the sum of _all_ processes of construction and disintegration in the living organism. It is apparent that equilibrium of metabolism occurs when assimilation and dissimilation are equal. The formula A : D, that is, the relation of the sum of all assimilation to the sum of that of all dissimilative processes, is a factor of fundamental importance in the study of the course of the vital processes, for upon its value depends individual vital manifestation, and, in fact, the continuation of life. I have, therefore, designated the formula A = D “_Biotonus_.” The equilibrium of metabolism would then be characterized by the biotonus[42] of a living organism being equal to _one_. This would be the metabolism of rest of a system, whilst its metabolism of stimulation would consist in an alteration of the _biotonus_. But is this state of living substance strictly speaking ever realized?