This study was carried out to test the biological actions of androgen on both male and female fowls. As androgen "Amolisin" was used; the "Amolisin" is oil solution of testosterone acetate. Fourteen fowls (8 females and 6 males), were used. Each fowl was given 50 mg of "Amolisin" during a period of 20 days by daily subcutaneous injections. The results of the investigations may be summarized as follows : Combs and wattles became enlarged, above all female combs extremely larger. Remarkable changes of the difference in their living weights could not be found between treated fowls and controls. After 5 or 6 days from the beginning of this treatment, the arrest of laying occurred in female fowls. Then after two weeks' injections they began to crow and behaved as if they were active males. They also imitated the male gestures of copulation to the control fowls. In anatomical observations, "Gefassreicher Korper (nach Liebe)" could not be found out in female fowls. Injected cocks' "Gefassreicher Korper" became more intensive in their reddish color. Testes, ovaries and oviducts decreased exceedingly in weight and size. Under histological observations, the hormon e accomplished its effect upon the comb primarily by stimulating the cells of the intermediate layer of loose connective tissue whose fibers were widely spread apart by a matrix of mucoid; especially the comb of injected hen had an intermediate layer much highly developed. In the "Gefassreichem Korper", loose connective tissue was found to have entered into marrow core tissue, and lymphatic sinuses were filled with lymph fluid. According as the shrinkage of the testis, the seminiferous tubules were reduced markedly in size and spermatogenesis was inhibited. Under the influence of androgen, atrophy of endometrium and oedema of the circular layer of myometrium in the uterus portion of oviduct occured. The changes induced in the albumen secreting portion of oviduct were almost of the same results as uterus portion. The characteristic changes which occurred in the anterior lobe of pituitary by injection of androgen, were degranulation of the basophiles and decreasing their cells in number and size. On the other hand, the results on the acidophiles had only a tendency of slightly increasing, but they were not so clear as basophiles. In these reactions females appeared to be more responsible to androgen than males.