Active extramedullary hemopoiesis developed in liver of adult female mice by blood depletion of splenectomized animals. Observations were made on adult C(3)H/He mice, males and females of 6 weeks and 6 months old. The mice were splenectomized and the blood was depleted by 0.6ml/once 20days after splenectomy. The hematocrit value decreased to about 30% at 2days after blood depletion and recovered to near normal level after 10days. No significant differences were observed in the hematocrit value in all the animals observed. Reticulocyte counts increased to 20-28% 4-6days after blood depletion with rapid decrease for the following 4days and then returned to the normal level 25-30days after depletion. The maximum level of reticulocyte counts was highest in aged females(28% ) and lowest in young males(20% ). The number of hemopoietic foci in liver observed on HE stained tissue sections showed a marked difference in each group. Male animals, both young and aged, developed no hemopoietic foci. Females developed many foci in liver, higher values in aged females than young ones. Discussion was made on the reason why aged females developed a large number of foci in liver.