Aims/hypothesis. The importance of different antioxidative enzymes for the defence of insulin-producing cells against the toxicity of nitric oxide (NO) was characterised in bioengineered RINm5F cells. Methods. RINm5F insulin-producing cells stably overexpressing glutathione peroxidase (GPX), catalase (CAT) or Cu/Zn superoxide dismutase (SOD) were exposed to S-nitroso-N-acetyl-d,l-penicillamine (SNAP), sodium nitroprusside (SNP) and 3 morpholinosydnonimine (SIN-1), which generate both NO and reactive oxygen species, and to the polyamine/NO, complex DETA/NO which generates NO alone. The viability of the cells was tested by the MTT assay. Results. Overexpression of antioxidant enzymes provided significant protection against the toxicity of SNAP, SNP and SIN-1, with an individual specificity related to their chemical characteristics, but was without effect upon the toxicity of DETA/NO. Cells overexpressing GPX were well protected against SNP and SNAP, while CAT was most effective against SIN-1. SOD overexpression provided less protection against the toxicity of SNAP and SNP than overexpression of GPX but was more effective in protecting against SIN-1. Co-incubation of cells with NO donors and hydrogen peroxide or hypoxanthine and xanthine oxidase showed an overadditive synergism of toxicity. Conclusion/interpretation. The results emphasise the importance of a synergism between NO and reactive oxygen species for pancreatic beta-cell death. Such a synergism has also been observed after exposure of beta cells to cytokines. The component of the toxicity that is mediated by oxygen radicals can be suppressed effectively through overexpression of CAT, GPX or SOD or both. [Diabetologia (1999) 42: 849–855]