Uranium toxicity has been analyzed primarily by investigating its radioactive properties. However as the densest naturally occurring element it also exerts chemical toxicity like other heavy metals. We have been investigating the reproductive chemical toxicity of uranium in female mice. In vivo exposure of mice to uranyl nitrate in their drinking water results in the selective significant reduction in only primary follicles in the ovary and increased uterine weight. These outcomes are typical estrogenic responses. To assess putative estrogenicity of uranium we used the estrogen sensitive Mcf-7 breast cancer cell E-Screen bioassay and preliminary data suggests that uranium, similar to many other heavy metals, is estrogenic. Here we propose to further characterize the putative estrogenicity of uranium. Our working hypothesis is that uranium stimulates estrogenic responses in vitro and in vivo via estrogen receptor alpha, which will be tested in 2 specific aims: 1) Use the Mcf-7 cell E-Screen assay to analyze uranium's estrogenic potency, receptor specificity and effective concentration, culture ovarian theca/interstitial cells to examine the impact of uranium on androgen production; 2) Conduct in vivo experiments in intact and ovariectomized mice to assess uranium's impact on reproductive organs and endocrine measures. NAU is located on the southwest perimeter of the Navajo Nation where until the early1990's there was active uranium mining. Unremediated mines and ore processing sites have contaminated drinking water sources with uranium at ppb to ppm levels. Data is very limited but shows that reproductive cancers in Navajo teenage girls are 17 times more frequent than in age-matched U.S. teenagers. Our long range goal is to address if it is possible that chronic consumption of uranium-contaminated water and/or in utero exposure, similar to the effect of diethylstilbestrol exposure in utero, has contributed to the exceedingly high incidence of genital and reproductive cancers in Navajo teenagers by investigating in our laboratory the putative estrogenicity of uranium in vitro and in vivo.