Tamoxifen as an anti-tumour agent: role of oestradiol and prolactin.

Four-day cyclic rats fed 7, 12-dimethylbenz(a)anthracene (dmba) (20 mg) at 50 days of age had peak prolactin, oestradiol and uterine wet weights at pro-oestrus. Tamoxifen (50, 200 and 800 mug daily), administered to ovariectomized rats, produced significant (P less than 0 X 05) decreases in oestrogen-stimulated prolactin levels but was unable to reduce prolactin to control values. Tamoxifen (12 X 5, 50 and 200 mug daily) produced decreases in size in DMBA-induced rat mammary carcinomata in intact rats although some tumours did not respond to therapy. The ability of the pituitary to produce prolactin was not impaired. Decreases in uterine wet weights and peripheral oestradiol levels occurred during tamoxifen treatment.