Methoxyacetic acid (MAA) is the toxic metabolite of ethylene glycol monomethyl ether (EGME), a known testicular toxicant. Apoptosis is thought to be the cause of germ cell death induced by MAA treatment, but the precise mechanism is unknown. The apoptotic initiation pathway known as the Fas system is a likely candidate for this mechanism. The Fas system is mediated by the cell surface proteins Fas receptor (Fas) and Fas ligand (FasL). In the current study the role of the Fas system in MAA induced apoptosis was studied by comparing wild type mice to mutant gld mice, which have a non-functional FasL. Wild type and gld mice were treated with 300 mg/kg MAA and then killed at either 6 or 12 hours after treatment. Germ cell apoptosis was assessed in control and treated animals by DNA fragmentation in situ by terminal deoxy-nucleotidyl transferase-mediated digoxigenin-UTP nick end labeling (TUNEL) of testis cross sections. Fas and FasL expression was measured in control and treated animals using RT-PCR. TUNEL staining illustrated lower baseline germ cell apoptosis in the gld mice and different patterns of apoptotic induction in the wild type and gld mice after MAA treatment. RT-PCR detection of Fas and FasL expression in wild type mice demonstrated an induction following MAA treatment. This induction was absent in the gld mice following MAA treatment. All of the results suggest that the Fas system is an important mechanism of apoptotic initiation in the wild type mice. However, the results also suggest that the Fas system in not functional in the gld mice and instead there is an alternative apoptotic pathway in use in these animals.