In 1931 we1 recorded data on the concentration of serum sulfate and of blood urea in cases of renal insufficiency of varying degrees. At that time we stated specifically that the concentration of serum sulfate may be increased in a case of early renal insufficiency while the concentration of blood urea is within normal limits. However, we now believe this fact may be more precisely stated by saying that the concentration of serum sulfate may exceed the normal limits in cases of mild, rather than early, renal insufficiency. When there is definite renal insufficiency, the concentrations of both blood urea and serum sulfate will usually be increased. Why the value for serum sulfate is increased when the concentration of blood urea is normal is a phenomenon we cannot explain. The functional activity of the kidney is so complex, however, that a moderate increase of any one substance in the