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The effects of medroxyprogesterone acetate (MPA) on precancerous mammary hyperplastic alveolar nodule (HAN) formation in mice were studied as a possible step to evaluate the anticarcinogenic role of MPA in mammary tumors. One month of subcutaneous pellet implantation of MPA to 6-7 month-old SHN breeding mice resulted in marked decrease and increase in the numbers of HAN and ghosts (the remnants of regressed HAN), respectively. The number of HAN still tended to be smaller in the experimental mice than in the control even 1 month after removal of MPA pellets which had been implanted for 2 months. Serum prolactin level at autopsy was apparently decreased and prolonged estrous vaginal smears observed in the control were replaced by diestrus in the experimental mice. All results indicate that MPA can lastingly inhibit HAN formation in mice, and suggest that MPA acts on mammary glands both directly and indirectly through its antiestrogenic effect.

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