The uteri of rabbits and cats have been analyzed for sodium, potassium, and chloride, usually after various preliminary treatments with estrogen and progesterone. These tissues contain less potassium (50–80 meq./kg.) than striate muscle and more sodium (75–118 meq./kg.) than other highly cellular tissues. It appears that this cation composition can be attributed in part to a relatively large extracellular fluid volume (EFV).Various methods have been used to estimate the EFV in these tissues. The radiosulphate space (in vitro) does not appear to be reliable as a measure of extracellular space. The chloride space varies, but exceeds 400 ml./kg. in all cases, and in some approaches 700 ml./kg. Inulin space (in vitro) is about 60% of the chloride space, which in turn is usually smaller than the sodium space. The chloride space appears to provide the best approximation to the EFV since its volume of distribution rarely exceeds the sodium space, and since chloride (but not sodium) can be removed completely on leaching in isotonic sucrose.Calculated cellular potassium concentrations are as high as or higher (150–210 meq./l.) than in striate muscle. Apparently the low total tissue potassium concentration is a consequence of the large EFV.Appreciable quantities of sodium (20–50 meq./l.) reside outside of the chloride space in most cases, presumably in cellular water. Furthermore, a residue of sodium remains in uterine tissue after leaching in isotonic sucrose or choline chloride. With appropriate leaching procedures, an initial rapid depletion of tissue sodium is followed by a period of relatively slow loss, indicating derivation of sodium from at least two separate tissue spaces. Equilibration in isotonic potassium chloride causes nearly complete equilibration of potassium and chloride throughout tissue water, but does not remove residual sodium, suggesting chemical binding rather than Donnan distribution as the mechanism of sodium retention.The effects of estrogen and progesterone on the concentrations of cations in uterine cells are shown to be relatively small. Estrogen causes expansion of the cellular compartment relative to the extracellular space in both rabbit and cat and decreases the concentration of cation (per liter of tissue water and per liter of intracellular fluid). Progesterone treatment, given after estrogen, interfered with the ready entrance of chloride into the cellular space of rabbit uterus exposed to isotonic choline chloride. Cat uterus was not so affected, there being very little penetration of chloride even after estrogen alone.