Studies of severe reactions in hyperpyrexia are highly suggestive of shock, and it is the thesis of this paper that the severer reactions are actually true shock. Although there have been numerous investigations into the theory of shock, no one theory has been adduced that is satisfactory to all workers in this field. Recent studies, however, have stressed the importance of a diminished volume of blood and an increased vascular permeability in the production of the shock syndrome.1 In fact, Moon1b has defined shock as "the disparity between the volume of blood and the volume-capacity of the vascular system." He has stated that shock may result either from loss of blood or fluid or from atony or dilatation of the vascular walls, especially of the capillaries and venules, combinations of these two factors being the rule. According to this concept, increased capillary permeability, with leakage of plasma through