Leydig cells undergo apoptosis in response to the cytotoxin ethane dimethanesulfonate (EDS), with numbers declining at 12-18 h and maximal apoptosis at 24 h postinjection. The Bcl-2 family members, Bcl-2, Bcl-xl, and Bax, appear not to be involved in this process. To further investigate this phenomena, a single dose of EDS was administered to adult rats to induce the killing of Leydig cells. The interstitial cells were examined up to 3 days after EDS administration by Western blot analysis for the Bcl-2 family members (Bak and Bcl-w). Western blotting showed that Bak expression in the interstitial cell preparations was unchanged after EDS, and immunohistochemistry showed that it was not up-regulated in Leydig cells in response to EDS. Bcl-w expression in the Leydig cells and interstitial cell preparations was unchanged until 48 h when it became undetectable, suggesting that Leydig cell-associated Bcl-w is not involved in initiating apoptosis. We also investigated the role of the Fas system in Leydig cell apoptosis. Both Fas receptor and Fas ligand protein levels increased after EDS, peaking at 12-18 h and declining thereafter. Fas receptor and ligand were shown by immunohistochemistry to be present in Leydig cells, and after EDS all Leydig cells became strongly positive for both proteins. The intensity of staining increased in the early stages of apoptosis and decreased as the nuclear morphology became more fragmented. These data suggest that Bcl-2 family members are not involved in Leydig cell apoptosis after EDS administration. However, up-regulation of the Fas system does occur, implicating activation of Fas receptor in the induction of Leydig cell apoptosis.