The laboratory has interests in male reproductive biology and toxicology. The male sex steroid hormone, testosterone, which maintains the male phenotype, is produced primarily by Leydig cells in the testis. However, the concept that ‘androgen is male and estrogen is female’ is no longer tenable because estrogen receptors (ERα and ERβ) are localized, along with androgen receptors, to male reproductive tissues, including Leydig cells. Data from transgenic mice lacking one or both ER subtypes support the hypothesis that estrogen has a regulatory role in male reproduction. Moreover, there is growing public concern that chemicals in the environment (food, air, water), which have estrogenic properties, may have adverse effects on reproductive health. These compounds alter the endocrine profile by acting through steroid hormone receptors and are designated endocrine disruptors. We will engage in the investigation of the role of estrogen, the ER, and the effects of endocrine disruptors on Leydig cell differentiated function and male reproduction.