Expts. are described to compare the responses of blood pressure to increased intra-abdominal pressure in animals of small abdominal areas with those of animals of large abdominal areas. Animals of small abdominal areas are more sensitive to given intra-abdominal pressure increases than animals of larger abdominal areas to the same amount of pressure increase; and animals with small abdominal areas go into circulatory failure and death sooner than animals of larger abdominal areas at given intra-abdominal pressures (10-30 mm. Hg.). Changes in respiratory activity in the thorax are described, showing that the upper thorax becomes more active as intra-abdominal pressure is increased, while the lower thorax becomes inactive and fixed, apparently because of displacement of the diaphragm towards the thorax, which raises the intrathoracic pressure. Changes in the electrocardiograms show axis deviation due to mediastinal shift, and suggest that conduction across the myocardium is interfered with (via vagal influence), evidenced by low voltage and notching of QRS complex. Section of the vagi causes an improvement in conduction.