Discussing experimental psychiatry Kraepelin observes that lack of comprehension of the experimental test, lack of ability to execute it, lack of interest, of co-operation, and of endurance, all conspire to increase the task of the experimenter and to modify the value of the results. The consequent demand for trustworthy experimental methods, which without too complicated technique and too unusual demands on the patient shall yield information of significant variations from the normal in quantitative terms, voices at once the need and the embarrassment of the experimental psychiatrist. The determination of the electrical conductivity of the skin is undoubtedly one valuable method of approach in this search after quantitative information, and this paper is the result of some months' investigation which I carried out at the Maudsley Hospital, when, through the kindness of the Medical Superintendent, Dr. E. Mapother, I had put at my disposal cases of psycho-neuroses and early psychoses.