It has been proved by many investigators that the adrenocortical function is closely interrelated with the ovarian function. It is believed that the estrogenic hormone stimulates the adrensal cortex either directly or indirectly by way of the pituitary gland. The authors have intended to observe the adrenocortical activities by counting the circulating eosinophils in the groups of rats to which 3 i.u., 6 i.u. and 9 i.u. of estrogenic hormone (Estradiol Benzoate, U.S.P.) were subcutaneously administered respectively. The results obtained in conjunction with the stages of their sexual cycles, and dose of single injection and the times of administration of the hormone, were as follows. (1) Circulating eosinophils were markedly diminished in number as -30.2%(-17.9% to -41.0%) average 6 hours after the injections. (2) There was no significant difference observed in the changing rates of eosinophils in each stage of sexual cycles. (3) The rate of decrease of cosinophils by a single dose was most marked in the groups to which 6 i.u. of the hormone was administered. (4) The rate of decrease of cosinophils evaluated by frequency of hormone administrations showed a tendency to diminish as the frequency increasesd, and the average rates of decrease were -34.1% (-48.0% to -18.1%), -11.3%(-33.2% to +13.6%) and -18.8%(-36.7% to +1.8%) in the groups to which 3, 6 and 9 i.u. of estrogenic hormone were administered 5, 10 and 15 times respectively.