For the immature uterus the water, potassium, sodium, and chloride (per kg wet tissue) are 838 g, 101 mEq, 87 mEq, and 67 mEq, respectively; for isolated myometrium under estrogen domination they are 799 g, 109 mEq, 79 mEq, and 67 mEq, respectively; for isolated myometrium under progesterone domination they are 792 g, 111 mEq, 83 mEq, and 72 mEq, respectively. Potassium contents are higher than those reported previously by other investigators, owing to a technique of preparation that avoided leaching of K from the tissues. Extracellular spaces estimated by distributions of inulin are 327 ml/kg for immature uterus, 322 ml/kg for estrogen-dominated myometrium, and 312 ml/kg for progesterone-dominated uterus. There is little statistically significant difference between extracellular spaces or ion content of the different hormonally affected uterus or myometrium. In estrogen-dominated myometrium, 14 mEq/kg of K and 29 mEq/kg of Na are unextractable in neutral isotonic sucrose. In progesterone-dominated myometrium, they are 4 and 9 mEq/kg, respectively. After correction for unextractable fractions, the sum of intracellular Na, K, and Cl in estrogen-dominated myometrium is in fair agreement with the total osmolar concentration of serum, but in progesterone-dominated myometrium it is still greater than the total osmolar concentration of serum.