Biological thiols are known to play an important role in the detoxification of xenobiotics, including chemical carcinogens. To determine the influence of cellular thiols on carcinogen induction of hepatic DNA damage in the rat, diethylmaleate (DEM) administration was used to deplete intracellular glutathione (GSH). The effects of administration of the synthetic thiols, N-acetylcysteine (NAC) and alpha-mercaptopropionylglycine (alpha MPG), on the induction of DNA lesions were also examined. Pretreatment with DEM reduced liver GSH levels by greater than 70%. As assessed by the technique of alkaline elution, subsequent administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) resulted in DNA damage 4 h post MNNG treatment which was 4- to 8-fold greater than that induced in the livers of rats treated with MNNG alone. However, DEM pretreatment had little effect on the extent of DNA damage induced by methylnitrosourea (MNU). DEM alone did not cause any measurable DNA damage. Pretreatment with alpha MPG or NAC reduced MNNG-induced DNA damage by as much as 77%. In contrast, MNU-induced DNA damage was increased by alpha MPG treatment whereas NAC treatment was without effect. These results indicated that in the rat liver, the activity of some DNA alkylating agents may be modulated in varying degree by the concentration of intracellular thiols. These data support the notion that thiols play an important role in protection against carcinogen damage, and that synthetic thiols such as alpha MPG and NAC may be useful as anti-carcinogenic agents against certain carcinogens.