Ultracentrifugal analyses at a density of 1.21 of serum lipoproteins in the nephrotic syndrome of rats induced by the intravenous injection of NTS obtained from rabbits, showed greatly increased concentration of all lipoprotein components but chiefly in the lower density fractions. The amount of change was not directly correlated with the duration of the disease, but was generally comparable to its severity. Changes observed in the rat were similar in type to those in the nephrotic syndrome of children.