Meishan embryos transferred to recipient females on day 2.5 are larger, contain greater numbers of trophectoderm cells, and secrete greater amounts of estradiol-17β when gestated in a Yorkshire compared with Meishan uterus to day 12. Additionally, placentae of Meishan conceptuses are larger when gestated in a Yorkshire compared with Meishan uterus throughout gestation. Embryonic estradiol-17β secretion during elongation on day 12 to 13 of gestation is temporally associated with endometrial secretion of growth factors, including IGF-I, which has been shown to increase mitotic rate in the trophectoderm of pig embryos. This experiment was conducted to determine if estradiol-17β administration to Meishan females at the time of conceptus elongation would increase placental size at term. Meishan females (n=12) were checked twice daily for estrus (0700 and 1900), and each bred to a Meishan boar at 0 and 24 h after the onset of estrus (day 0). Females were randomly assigned in equal numbers to receive injections of sesame oil (VEH) starting on day 12 (CONTROL), 1 mg of estradiol-17β in VEH starting on day 12 (E212), or 1 mg of estradiol-17β in VEH starting day 13 (E213). The injections were initiated at 0700 or 1900 (corresponding to the time of day they first exhibited estrus) and continued at 6-hour intervals for 48 hours, resulting in 8 mg of estradiol-17β given in eight injections. Pregnant females were killed on day 112 of gestation and ovulation rate, litter size, implantation site length, fetal weight, crown-rump length, placental weight, and placental surface area were quantified. There were no differences among E212, E213, and CONTROL females in ovulation rate or litter size, which averaged 16.3 ± .7 and 11.8 ± .7, respectively. Fetal weight and crown-rump length were not different (P>.10) among E212, E213, and CONTROL females, averaging 802 ± 26 g and 24.3 ± .3 cm. Placentae were markedly heavier (176 ± 14 and 174 ± 16 g vs. 134 ± 10 g, P