Adult mongrel dogs were given a lethal dose of E. Coli endotoxin by rapid intravenous injection. Total serotonin levels in the serum fell rapidly, with the concentration in the portal vein and pulmonary artery significantly exceeding that in the femoral artery within the first minute after injection. Small rises in plasma serotonin were found in some of the animals. These changes were coincident with a sharp fall in the number of circulating platelets, with striking changes in platelet morphology, and with the initial fall in blood pressure.