Hemodynamic alterations were produced in dogs by injecting meningococcus toxin. Hypotension and a decrease in cardiac output occurred without significant reduction in blood volume. These changes were accentuated by small hemorrhage. Increasing the blood volume above normal led to marked increase in cardiac output with slight increase in arterial pressure. The arterial pressure was markedly increased by the administration of levarterenol but large doses were required. Cortisone did not modify the effect of meningococcus toxin on the circulation. Meningococcus toxin apparently produced its effect by increasing the volume of blood in small blood vessels.