The following thesis is concerned with the histological examination of some of the endocrine glands obtained from cases of mental disease; these have been compared with a series of glands obtained from cases of presumed normal mentality. The following conclusions have been drawn.; 1) In a high percentage of cases of mental disease there can be demonstrated an increase of fibrous tissue throughout the endocrine glands. It is important to note, however, that in normal cases there is also a tendency for the fibrous tissue to be increased though the extent of fibrosis is certainly less than in psychotic material. 2) In schizophrenic cases there is marked tendency for the nuclear chromatin to be reduced; this is especially marked in the testes of male cases. This is not a constant finding and it should be pointed out that this condition may occur in any chronic disease. The etiology of chromatin deficiency is by no means certain. 3) In schizophrenic states there are commonly marked departures from the predicted endocrine weights. 4) The cardio- vascular system tends to be underdeveloped in schizophrenics. In manic- depressive states the cardio- vascular system is L:téll developed and may be hypertrophied. There is a curious fine atheroma found in many cases of schizophrenia; this affects the aorta in quite young cases; the cause is obscure but may be toxic. 5) In manic states there is usually a well marked preponderance of eosinophil cells in the anterior lobe of the pituitary. In depressive states the basophils tend to predominate. In depressive states it is the rule to find pituitary glands which are well above the average predicted weight. in manic states the weight is average. 6) In epilepsy there is a fairly constant increase of fibrous tissue in the anterior lobe of the pituitary. There is a tendency for the nuclear chromatin to be reduced in many of the endocrine glands. A general criticism of the above results is that there are no constant findings which can be related to any definite mental state; this however could hardly be expected. The etiology of mental abnormality must, from the nature of the problem, be very complex. It would be irrational to expect to find a pathology which could be described entirely,- in terms of either morbid anatomy or psycho- pathology. Behaviour, be it normal or abnormal, is the overt reaction of an organism to its environment. The human organism is composed of a multitude of cells which are collected together into functional units; each of these units has to adapt itself to a local envi:. °on: ;nt . Each unit must be working in harmonious cooperation with the others if the whole organism is to be in a state of equilibrium. There is abundant evidence to show the importance of the endocrine gland unit in the biological reactions of all the higher animals. There is a very intimate relationship between the ductless glands and the nervous system; in fact it appears probable that the efficient working of the latter is entirely dependent upon a correctly balanced endocrine system. Histo -pathological methods of investigation have many obvious drawbacks; it is difficult to avoid aretfacts due to post -mortem change and distortion due to fixation; it is still more difficult to estimate the probable degree of physiological activity of the gland from the post-mortem appearances. The only reliable approach to the problem is biological. It is the writer's hope to continue investigations along these lines and to be able to throw some light on the functional activity of the endocrines in mental disease.