The investigation of the problems of metabolism has now become almost exclusively the domain of the chemical physiologist. Much valuable information regarding the method of utilisation of the food-stuffs which enter the body has been ascertained by a study of the excretions. The physiological chemist has, however, passed beyond the boundaries connecting the income and output of these substances. He now seeks to trace the different transformations and combinations which take place in the body, and to connect up all the links of the chain. These changes are intimately bound up with the individual cells, and their metabolism. Unfortunately our knowledge of the cell is at present very limited, but it will be seen how important even this scanty information is to the subject under discussion. It becomes more and more evident every day that pathological changes in the tissues and cells of the body must be considered not only from a morphological point of view, but also from the purely metabolic standpoint. A disturbance in the metabolism of the cell may in time make itself evident, but it is quite conceivable that such changes are taking place without any definite anatomical signs. On the other hand, a morphological change may produce only a very slight derangement of cell metabolism, so slight as to escape recognition. The study, therefore, of pathology with physiological chemistry for its foundation, offers a wide field for further investigation. The object of the present communication is twofold, namely, to correlate the known facts with regard to metabolism in the insane, and to emphasise the importance of studying cellular metabolism in its relation to pathological disturbances.