During the last year we have been attempting to increase our understanding of the significance of the various dilution methods used to measure the extra-cellular fluid. When the dilution methods were first proposed to obtain a measure of the extracellular fluid volume, it was hoped that an agent could be found which would distribute uniformly and exclusively throughout this compartment, which would approach equilibrium rapidly, and which could be measured by simple and accurate procedures. We have turned our attention chiefly to the methods themselves using dogs and non-edematous adults as subjects.