The cardiac output during the shock phase of hemorrhagic fever was very low. Increases in mean arterial pressure resulted from therapy with albumin or norepinephrine. With the former, this increase was associated with an increased cardiac output; with the latter, an increase in peripheral resistance. During the hypertensive phase, patients were found to have an increased cardiac output if they were also oliguric, and a relatively normal cardiac output if diuresis had begun. Some of the hypertensive patients presented the clinical syndrome of the "hyperdynamic state"; this occurred almost exclusively during the oliguric phase and was associated with high cardiac output and metabolic acidosis.