The primary basis of 5-nitro-3-thiophenecarboxanilides (NTCAs) direct-acting mutagenicity in Salmonella typhimurium appears to be the reduction of the nitro function to the corresponding hydroxylamine via diamagnetic and free radicals intermediates. In Ames test conditions, mutagenicity of NTCAs may be partly attributed to the action of reactive oxygen species and other radicals generated during the bioreductive process. The nitro anion radical seems to be particularly susceptible to react with oxygen and generate superoxide, as shown by the inhibitory effects exerted by superoxide dismutase on genotoxicity by most NTCAs in the study. On the other hand, hydroxyl radical-trapping agents such as mannitol, the enzyme catalase and other scavengers as 3-tert-butyl-4-hydroxyanisole (BHA), and vitamins C, A, and E showed weak inhibitory potency, and rather increased the mutagenic activity of some NTCAs. Our results contribute to the mechanistic understanding of genotoxic activity of NTCAs.