THE description of a line of transplanted mammary fibroadenomata in rats was presented in a preceding paper from this laboratory (Millar and Noble, 1954).Observations by us and others have shown that this type of tumour is more easily propagated, grows more rapidly and usually shows greater epithelial development in the female host than in the male. In addition, castration in males and females increases and decreases respectively the growth energy of transplanted mammary fibroadenomata in rats. In order to elucidate further the nature of the hormonal control of these benign tumours, experiments have been carried out to determine the effect of administered pituitary, adrenal and sex hormones on their growth and morphology.The effect of estrogens on the growth of mammary fibroadenomata in rats has been studied by several workers with somewhat conflicting results. Heiman and Krehbiel (1936) found no consistent tumour response to administered estrogens. However, estrogen (theelin) plus growth hormone (Antuitrin S) or gonadotrophic hormone (Antuitrin G) increased the number of tumour takes in intact rats, although none of the individual preparations were active. Antuitrin S alone was effective in increasing the takes in castrated males and females, while estrogen and growth hormone were again effective only when injected simultaneously. In these experiments tumours of the 13th to 22nd generations were used.Estrogen administration to rats bearing auto-and homo-transplants of spontaneous tumours produced marked tumour epithelial proliferation and increased growth rate (Heiman, 1940b). Mohs (1940)