Experiments to measure the changes in water and electrolyte distribution in euthyroid and hypothyroid dogs as a result of an infusion of HCl. This paper hypothesizes that the physico-chemical state of the connective tissue ground substance is altered by changes in the hydrogen ion concentration of the interstitial fluid. Since a base infusion increased the extracellular water and the interstitial water in the hypothyroid, as compared to the euthyroid, an acid infusion should yield comparative changes in the opposite direction. Of all the data taken, only the plasma water and interstitial water volumes showed a significantly different change between the two groups of animals. When these results are compared to those presented by Dorr for NaHCO sub 3, it is found that the volume of interstitial water in the hypothyroids increases relative to the euthyroids after NaHCO sub 3 is administered, and decreases relative to the euthyroids after the infusion of HCl. 8 pp. Ref.