Estrogenic partial effect of norethisterone enanthate in relation to tumor induction in rat mammary gland.

The tumorigenic effect of norethisterone enanthate (NE) on mammary glands in rat and mouse may be ascribed to its own inherent estrogenicity, which is especially pronounced in these species. Because of the estrogenic partial effect, NE stimulates tubulo-alveolar growth of rat mammary gland intensively as does an estrogen-progesterone combination. Although prolactin is known as one of the essential factors needed for development of rat mammary gland, NE causes only a slight increase in serum prolactin level in the rat. In contrast with rat and mouse, NE exerts only a progestational effect in the rabbit. Since the estrogenicity of NE has not been found in many clinical studies, the estrogenic partial effect of NE seems to have no significance in humans. Such a species difference is finally due to the fact that the optimal dose ratio between estrogens and progesterone for maintenance of physiological functions of target organs is quite different from species to species.