1,3-Dinitrobenzene (1,3-DNB) but not 1,2-dinitrobenzene (1,2-DNB) or 1,4-dinitrobenzene (1,4-DNB) is a potent testicular toxicant in rats. In vitro metabolism studies have established that 1,3-DNB is reduced to 3-nitroso-nitrobenzene (3-NNB), 3-nitrophenylhydroxylamine (3-NP) and 3-nitroaniline (3-NA) in testicular cytosol and Sertoli cell cultures. To establish a potential role for endogenous glutathione (GSH) in the detoxification of the electrophilic metabolite 3-NNB, we examined the chemical reaction of this compound with biological thiols, including GSH. The effect of pH and thiol concentration upon the reaction were studied. The reaction of GSH with 3-NNB was complex and gave three distinct products. These were identified as 3-NP, 3-NA and a glutathionyl derivative containing a covalently linked S-N bond. The hydroxyl amine and the amine were isolated and fully characterised. The glutathion-S-yl derivative was characterised in solution by proton NMR (400 MHz), infra-red and mass spectroscopy to establish its structure as the semimercaptal, N-(glutathion-S-yl)-N-hydroxy-3-nitroaniline (GSNOH-3NA). Similar reactions were performed with 4-nitrosonitrobenzene (4-NNB) to ascertain the reactivity of this chemical towards thiols. The addition of GSH to 4-NNB resulted in the rapid formation of 4-nitrophenylhydroxylamine (4-NP) and an adduct that was identified as the semimercaptal N-(glutathion-S-yl)-N-hydroxy-4-nitroaniline (GSNOH-4NA).