Female mice treated neonatally with the phytoestrogen genistein have multioocyte follicles, lack regular estrous cyclicity, and are infertile even after superovulation. To determine the cause of their infertility, we examined both oocyte developmental competence and timing of embryo loss. Eggs obtained by superovulation of genistein-treated or control females were equally capable of being fertilized in vitro and cultured to the blastocyst stage. However, if eggs were fertilized in vivo, retrieved at the pronucleus stage and cultured, there was a small reduction in the percentage of embryos from genistein-treated females reaching the blastocyst stage. When these blastocysts were transferred to pseudopregnant recipients, the number of live pups produced was similar to controls. Preimplantation embryo development in vivo was examined by flushing embryos from the oviduct and/or uterus. Similar numbers of 1-cell and 2-cell embryos were obtained from genistein-treated and control females. However, significantly fewer embryos were obtained from genistein-treated females on postcoital days 3 and 4. Initial histological analysis has demonstrated abnormal epithelial proliferation in the oviduct on day 2 of pregnancy that may contribute to an oviductal environmental defect. To determine if neonatal genistein treatment altered the ability of the uterus to support implantation, blastocysts from control donors were transferred to control and genistein-treated pseudopregnant recipients. The genistein-treated females did not have any normal implantation sites on day 8 of pregnancy, although a few abnormally small implantation sites were observed. Taken together, these results suggest that oocytes from mice treated neonatally with genistein are developmentally competent; however, both the oviductal environment and the uterus have abnormalities that contribute to the observed reproductive failure. Supported by the Intramural Research Program of the NIH, National Institutes of Environmental Health Sciences, Z01-ES-102405-01