Effect of ovarian hormones on the induction of 1-methyl-1-nitrosourea-induced mammary cancer.

The concomitant administration of 20 micrograms 17 beta-estradiol and 4 mg progesterone/injection (5 days/week) over a 40-day treatment period beginning 10 days after methylnitrosourea (MNU) treatment was as effective as ovariectomy in inhibiting mammary cancers (an 84% reduction from non-hormone treated rats). The primary action of the hormones must have been directed at preneoplastic cells since the hormones were not given until 10 days after MNU (a direct-acting carcinogen with a short half-life).