Perfusion pressure reduction to the small intestinal vascular bed for 2 hours to followed by characteristic villous damage and a general cardiovascular derangement. In an extensive series of experiments on cats it was demonstrated that the cardiovascular derangement could not be ascribed to any pooling of blood and/or fluid in the small intestine. Cardiotoxic material has been demonstrated in the cat intestinal venous blood in in vivo and in vitro experiments using working rat hearts and isolated rabbit papillary muscles. A relationship between the extent of the villous damage and the appearance of cardiotoxic material in the intestinal venous blood has been found. The cardiotoxic material seems to consist of at least two heat stable fractions. One is water soluble with a molecular mass between 500 and 1,000 daltons and the other is lipid soluble with an unknown molecular mass.