In an effort to determine the efficacy of cortical extract in the prophylaxis and the treatment of shock, the extract was given to animals subjected to shocking procedures and to human beings having major operations of a magnitude apt to produce shock. In animals, shock was produced by hemorrhage and by massage of the intestine with the animal under ether anesthesia. In experimental shock produced by hemorrhage cortical extract exerted a definite but slight tendency to decrease the severity of shock, particularly when the extract was given with fluids (glucose and electrolytes). Fluids alone, i.e., without cortical extract, did not significantly prevent the fall in blood pressure although the actual survival time following hemorrhage was increased. When cortical extract was given an hour or two before institution of intestinal massage (to produce shock), the average drop in systolic blood pressure after forty minutes of massage was only 19.5 mm., contrasted with 35.3 mm. in animals not receiving extract prophylactically. Moreover, when hemorrhage of a constant rate was instituted ninety minutes after intestinal massage was begun, there was a survival time of thirty-six and five-tenths minutes in animals receiving cortical extract, contrasted with a survival time of only twenty-one minutes in animals not receiving cortical extract. When glucose and electrolytes were given in addition to extract, the effect was still more prominent, there being no drop in blood pressure after forty minutes of massage and an average of only 7.5 mm. after fifty-five minutes of massage. In other words, the beneficial effect of cortical extract and fluids (glucose and electrolytes) in prevention of shock was comparable to that which might be expected from plasma. When extract was given after shock had already been produced in animals very little beneficial effect could be demonstrated; there was little or no rise in blood pressure although further drop was slightly delayed. Falling drop determinations showed an increase in specific gravity of blood in shock, and a decrease after hemorrhage as reported by others. Cortical extract showed a definite but slight tendency to counteract the increase in specific gravity produced by shock. In human beings the use of cortical extract in the presence of shock was limited to a few cases due to the relative infrequency of shock, but in every instance the effect appeared to be beneficial. In order to determine whether or not cortical extract would exert a favorable influence in prevention of shock, the average pulse rate and blood pressure was computed in a series of fifteen patients receiving cortical extract prophylactically, and upon whom major operations of unusual duration and severity were performed. From the hospital records an equal number of operations (in which no cortical extract was given), with equal number of types and duration, were obtained and the average pulse rate and blood pressure likewise determined. In the patients receiving cortical extract prophylactically, the pulse rate averaged eight beats per minute slower, and the systolic blood pressure 12 mm. higher than in the patients not receiving extract. This suggests that the cortical extract tends to minimize the changes which might be interpreted as being changes preliminary to the development of shock. Observations on animals as well as patients showed that the extract was much more effective when given prophylactically, than when given after shock was already produced.