The biological activity of a single dose of tamoxifen in the adult ovariectomized rat.

1 The peripheral and central activities of tamoxifen were studied in the ovariectomized adult rat, up to 16 d after a single dose of 7.0 or 0.7 mg/kg. 2 Food consumption and body weight were decreased; only food consumption returned to control values after 16 d. 3 The weights of the uterus and of the uterine luminal fluid were increased for up to 8 d. 4 Serum follicle-stimulating hormone (FSH) concentrations decreased, but only significantly 1 and 8 d after 7.0 mg tamoxifen/kg. Serum luteinizing hormone (LH) and prolactin concentrations were elevated for up to 4 d. 5 Lordosis behaviour was absent throughout the period studied. 6 Within 3 d of administration, tamoxifen partially antagonized the oestrogen-induced changes in uterine luminal fluid, prolactin and LH secretion and lordosis behaviour. 7 Tamoxifen did not alter oestrogen-induced changes in uterine weight, food consumption and body weight. 8 The experiments demonstrate that tamoxifen is active for up to 16 d after a single intraperitoneal dose; oestrogen agonist, partial agonist and antagonist activities were demonstrated. The duration and type of activity depends upon the dose of tamoxifen and the target tissue response examined.