In this preliminary communication we wish to report briefly the course of anesthesia in three operations performed on human beings exclusively under the influence of an intravenous injection of magnesium sulphate. The object of this report is, at least for the present, essentially a theoretical one. These few observations, although far from being sufficient to demonstrate the practicability of this method of anesthesia, are completely sufficient to establish the theoretical question regarding the nature of the anesthesia produced by magnesium sulphate. Furthermore, the theoretical information thus obtained is an indispensable step in the development of the practical application of this method. In order to understand the question under discussion, a brief historical statement of the various phases in the progress of this investigation is necessary. In a series of experiments on the effects of intracerebral injections of solutions of various salts carried on some eighteen years ago on rabbits by