The effect of hormones on mammary tumours induced experimentally in animals are revised. It is stated that the effect on tumoural induction and/or growth may depend on their concentration and on the moment, in relation to animal exposure to the carcinogen, in which they are administered. The presently known action mechanisms are also indicated, bringing out the importance of the increased level of differentiation which high estrogen doses induce, as well as some effects of progesterone and pregnancy in the protection against experimental cancer and its promoters. The efficacy of therapeutic ablative operations--ovariectomy, hypophysectomy and adrenalectomy--and additive ones--antihormones--in tumor regression, is shown. Finally the molecular bases of hormonedependence of experimental mammary tumours is established, while trying to provide an integrated concept between: hormones, receptor, growth factors, oncogens and experimental carcinogenesis.