The purpose of this investigation was to explore the reason why nickel chloride enhances the cytotoxicity and genotoxicity of ultraviolet (UV) light, but not that of methyl methanesulfonate (MMS) in Chinese hamster ovary cells. The cellular glutathione content was increased by treatment with MMS or nickel, but not with UV. Post-treatment with nickel synergistically raised the cellular glutathione content in MMS-treated cells; this phenomenon was not observed in UV-irradiated cells. Preventing cellular glutathione induction by buthionine sulfoximine increased the cytotoxicity, the frequency of sister chromatid exchange and prolonged the cell cycle in cells treated with nickel or MMS plus nickel. Pretreatment with N-acetylcysteine, a glutathione precursor, increased the clonogenic survival of cells treated with UV plus nickel. In vitro assays indicated that nickel could inhibit oligonucleotide ligation and the repair synthesis of UV- or MMS-treated plasmids and glutathione could relieve nickel inhibition. These results suggest that the enhancement by nickel of UV cytotoxicity and genotoxicity may be due to its inhibition of DNA repair, whereas treating cells with MMS plus nickel increased cellular glutathione levels, which may help in neutralizing the toxicity of nickel. The results also suggest that the activity of gamma-glutamylcysteine synthetase, the rate-limiting enzyme in glutathione biosynthesis, may be increased by treatment with MMS, nickel and more so with MMS plus nickel.