These studies were carried out to investigate the effects of the antiestrogens, LY-117018 and tamoxifen, on implantation in ovariectomized or intact adult rats. A Quantity of 80 μg of LY-l17018 or tamoxifen was given to adult female rats on Day 1, 2, 3 and 4 of pregnancy and investigated the implantation sites on Bay 8 of pregnancy. The rats were ovariectomized at the first day of pregnancy and treated with various doses of LY-l17018 or tamoxifen together with progesterone daily from Day 2 to 8 of pregnancy and then investigated the implantation sites on Day 8 of pregnancy The results were summarized as follows; When a single dose of 80 μg LY-l17018 and tamoxifen was given during the first 4 days of pregnancy, the implantation was intesively inhibited in the pregnant rat treated with LY-l17018 on Day 2 (14.4±3.5%), 3(16.3±5.3%) and tamoxifen on Days 2 (17.4±4.6%), 3 (16.3±2.8%) and 4 (13.9±3.5%). LY-l17018 was apt to inhibit more potently the implantation than tamoxifen except on Day 4 of pregnancy In rats ovariectomized on Day 1 of pregnancy and treated continucusly with 12? r9 of LY-117018 and tamoxifen together with progesterone showed the highest implantation rate, compared with the rats treated continuously with different doses of the two drugs. The correlation coefficients between the dosage of drugs and implantation rate were r= 0.91 (LY-117018), 0.51 (tamoxifen), respectively, except treatment with 625 μg of the drugs. Tamoxifen was apt to stimulate the implantation more potently than LY-l17018 except groups treated with 625 μg of the two drugs.