Estradiol dipropionate and estrone have only a gynogenic action on the genital tracts of young opossums of either sex, contrasting in this respect with the male hormone testosterone propionate which exerts a dual effect. The most striking effects are exerted on the phallus and the epithelium of the urinogenital sinus. Müllerian ducts, although very immature, are definitely enlarged. Prostatic outgrowths are suppressed.