One of the concerns about MtBE contamination in drinking water is its potential impact on reproduction. Our development of a non-invasive assay for reproductive state in male mice provides an opportunity to investigate MtBE and related compounds. The assay consists of a testosterone ELISA and a way to apply it to ether-extracts of feces. We have recently completed work in which the fecal testosterone biomarker has been validated in laboratory studies and applied in one field investigation. In the current project, mice are gavaged with MtBE. Parameters to be studied include (a) fecal testosterone levels, (b) end point serum testosterone, (c) animal and testes weights, and (d) histopathology of the testes.