Buthionine sulfoximine (BSO) treatment significantly reduced testicular epididymal and vas deferens glutathione (GSH) levels in rats. Testicular levels of GSH were reduced by 20%, while epididymal GSH levels were reduced by more than 50%. BSO treatment correspondingly enhanced ethyl methanesulfonate (EMS)-induced dominant lethal mutations. EMS-induced resorption rates were doubled following BSO treatment. This effect was observed in mating wk 2 and 3 (d 8-19 following treatment), indicating effects on those germ cells which were in late testicular stages or were caput epididymal spermatozoa at the time of EMS treatment. The enhancement of the mutagenic action of EMS by BSO is restricted to the same time period (spermatid-spermatozoa transition, early epididymal maturation) as maximum sensitivity to the clastogenic action of EMS on male germ cells. The temporal pattern of EMS alkylation of rat spermatozoa correlated with the incidence of EMS-induced dominant lethal mutations. BSO depresses GSH in the male reproductive tract in a dose- and time-dependent manner. Perturbation of GSH in the male reproductive tract appears to influence chemical-induced germ cell mutations.