Several years ago1 we published a study of a case of diabetes insipidus in which, among other points, attention was directed to the question, whether any well marked changes could be detected in the blood when the water intake was greatly restricted or water taken at discretion. The patient had an enormous diuresis and a correspondingly great thirst, so that the conditions seemed unusually favorable for the inquiry. The conductivity of the serum and the relative volume of the serum and corpuscles were selected for study because the conductivity can be measured with great accuracy, and from the conductivities of the serum and blood the percentage volume of serum can also be obtained to a close approximation. Even when great changes were taking place in the rate of absorption, elimination and transportation of water, it was found that the two quantities measured altered only very slightly, although there seemed to