Evidence of hormonally induced sex reversal of the gonad has been furnished for cold blooded vertebrates and birds, but there is little known regarding such processes in mammals. Most authors attribute the gonadal changes in the freemartin to the heterosexual hormone. I n the fowl, Dantchakoff ('36) and Willier, Gallagher and Koch ( '37) obtained feminization of the testis by estrogens. I n fishes and amphibians in some instances the sexual differentiation of the testis (Foote and Witschi, '39 ; Puckett, '40 ; Berkowitz, '41 ; Saunders and Rugh, '42) and in other cases that of the ovary (Witschi and Crown, '37; Gallien, '38; Puckett, '40; Foote, '40; '41; Eversole, '41) could be reversed by the respective heterosexual hormone. I n the present communication the ability of male hormone to reverse to a certain degree the sex of the gonad is demonstrated in a mammal.
MATERIAL
Two young rats injected with testosterone propionate during June 1940 furnished ovaries the inner portions of which presented the histology of testes. This effect had never been noted before in rats treated in this laboratory with testosterone and testosterone esters. Various attempts were therefore made to reproduce this unusual result. Treatment of the Aaroii lMendelson research assistant.