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 (ESRs) and androgen receptors are both localized to the male reproductive tract, including Leydig cells. Data from transgenic mice lacking ESRs support the hypothesis that estrogen has a physiological role in male reproduction. However, there is growing public concern that chemicals in the environment (food, air, water), which have estrogenic properties, may exert adverse effects on reproductive health. These compounds mimic and/or antagonize steroid hormones, acting through steroid hormone receptors, and interfere with the function of the endocrine axis. Our major focus will be investigation of the role of steroid hormone receptors in mediating effects of environmental toxicants on Leydig cell differentiated function and male reproduction.