Introduction During the past three years methods of experimentation have been devised which render it possible to study pressure in the pulmonary circulation with a degree of readiness and an approach to normality unreached by earlier experiments. These methods have been described (1) (2) and have been tested in a series of published investigations (3) (4) and in a large amount of unpublished work. In the rabbit, cat, and monkey they permit observation of the pulmonary circuit under natural conditions of breathing and without encroachment upon the vascular bed. In the dog, owing to the depth and rigidity of the chest, artificial respiration must be used. The gross physiological disturbances which make up the curious entity called acute anaphylactic shock differ widely in different species of animals, and owing to the fact that the pulmonary circulation has been difficult of access, precise measurements upon it during the anaphylactic reaction are lacking.