Hemodynamic changes following the infusion of a number of intravenous fat emulsions are described. A prompt rise in portal venous pressure followed in seconds by systemic hypotension and subsequently by a rise in the pulmonary arterial pressure is associated with toxic preparations. These reactions are accompanied by acidosis and blood oxygen desaturation, and in several ways are similar to those which occur following incompatible blood transfusion and intravenous histamine injection. There is a good correlation between these reactions and the presence of large particles (over 6 µ) in the emulsion. When the administration of low molecular weight dextran precedes the infusion of the toxic fat emulsion, the reactions are modified and attenuated. Most of the major acute toxicity of a given intravenous fat preparation may be predicted from the results of such hemodynamic studies and from the distribution and size of the particles in the emulsion. Absence of acute hemodynamic changes, however, does not insure against the occurrence of chronic reactions following repeated infusions, which require different methods of investigation and prolonged observations.