Studies were made in 5 monkeys, 7 rabbits, and 33 cats of the effect of gram-negative endotoxin on aortic, pulmonary artery (PAP), and portal venous (PVP) pressures; and on changes in weight of a short segment of intestine. Studies of blood pooling were also made in 12 cats. The responses in these species were compared with those previously observed in the dog. Although variable degrees of hypotension developed at one time or another in all animals following injection of endotoxin, the early precipitous hypotension that characteristically occurs in the dog was observed only in the cat. However, in this species the dramatic fall in pressure could be ascribed to pulmonary vascular constriction and acute right ventricular hypertension and failure, and not to splanchnic pooling. PAP also became elevated in the monkey and the rabbit, but usually was of lesser magnitude and did not appear to explain the development of the relatively late hypotension that occurred in these species. The absence of significant inc...