The action of oestrogen overdosage has been studied by numerous investigators, and it was found that besides influencing the development of the sex organs, these substances exert a marked influence on the pituitary, the adrenals, the liver, &c. [Hohlweg, 1934; Wolfe, 1935 a, b; Selye, Collip, and Thomson, 1935; Selye, Harlow, and Collip, 1936; and Selye, 1938, 1939 a]. Since relatively few observations have been made up to the present along similar lines with chemically pure androgens, it appeared of interest to make a systematic study of the morphological changes produced by large doses of testosterone. As most of our previous work with the oestrogens was performed on female animals, we decided to use females for this work also, so as to make experimental conditions comparable. The findings of previous investigators concerning the action of androgens on various organs are rather contradictory, partly because they used impure preparations, and partly