This study has confirmed previous experiments which indicated that inorganic phosphate concentrations rise in body fluids after intestinal infarction. The phosphate clearance ratio is elevated in dogs after occlusion of the superior mesenteric artery, but is not as sensitive as indicator of the presence of a necrotic intestine as the serum or peritoneal fluid phosphate levels. The rise and fall of phosphate levels occur within the first 24 hours after infarction, similar to amylase levels in acute pancreatitis. The renal failure observed in dogs after ligation of the superior mesenteric vein is likely on the basis of hypovolemia and does not seem to result from hyperphosphatemia. Intestinal infarction should be suspected with laboratory evidence of hemo-concentration, marked leukocytosis, an inappropriate metabolic acidosis and elevated serum and peritoneal fluid phosphate levels. In dogs with normal renal function, an elevated urinary phosphate clearance ratio of greater than 0.20, as measured on a spot sample of urine and serum, is also highly suggestive of the presence of this condition, but its absence, by no means, excludes the diagnosis. With these simple laboratory tests, possibly an earlier diagnosis can be made, and timely operation can be an entirely intestine-saving procedure.