Adenosis-like lesions (ADL) were found in the upper vagina including the cervical region of ICR mice as early as 1 day after 5 daily injections of 200 micrograms tamoxifen (Tx) starting on the day of birth (200-micrograms Tx mice). ADL developed progressively from 5 to 30 days of age. Involution of the musculature and suppression of the gland genesis occurred in the uterus of 5- to 30-day-old, 200-micrograms Tx mice. The uterine abnormality resulted in a loss of type III collagen and laminin, and in an increase in fibronectin and type I collagen in the mesenchymal stroma of 15-day-old, 200-microgram Tx mice. Vaginal ADL and uterine myometrial involution were also encountered in 35-day-old mice given neonatal injections of 2-200 micrograms Tx, 200 micrograms clomiphene (Clm) and 200 micrograms nafoxidine (Naf), respectively. These changes were never observed in control and 0.2-microgram Tx mice. Ovariectomy performed at 10 days resulted in a reduction in the weight of uteri and the extent of ADL occupancy in Tx, Clm and Naf mice. Five daily injections of 0.1 microgram 17 beta-estradiol (E2) beginning at 30 days of age increased uterine weight and ADL occupancy extent in Tx (2-200 micrograms), Clm and Naf mice. The present findings suggest, therefore, that the antiestrogens act as an estrogen agonist to mouse uterus and vagina and that the induced ADL develops progressively depending on ovarian estrogen.