Dogs were made thrombocytopenic (platelet count less than 12,000) with serial intramuscular injections of goat antiplatelet serum (APS), and the hemodynamic response to 1 mg/kg of E. coli endotoxin administered intravenously was studied. Intramuscularly administered APS rendered dogs thrombocytopenic without major alteration in other blood elements or coagulation parameters. The response of normal dogs to endotoxin was a fall in systemic blood pressure and cardiac output (CO) with an increase in pulmonary artery (PA) pressure and an increase in pulmonary vascular resistance (PVR) to 600% of control with a minimal fall in mean left atrial (LA) pressure and pulmonary artery wedge (PAW) pressure. Thrombocytopenic dogs had an identical fall in systemic pressure and CO, but obliteration of the initial PA hypertensive response. LA and PAW pressures fell slightly as in normal dogs, and the obliteration of the PA hypertension was caused by an attenuation of the increase in PVR to a third of that of normals. Cinemicroscopic studies of the lung in vivo showed marked and prolonged slowing of the microcirculation following endotoxin administration in normal dogs. In thrombocytopenic animals slowing of the pulmonary microcirculation was brief and mild. We conclude that platelets are an essential component for the initial pulmonary hypertensive response to infusion of E. coli endotoxin in dogs.