The role of reactive oxygen species in diabetes and its complications are well known. Two therapeutic agents commonly used in the treatment of diabetes are the sulfonylureas gliclazide and glibenclamide. These drugs effectively reduce blood sugar in non-insulin-dependent diabetes mellitus, by augmenting insulin release. Gliclazide is known to be a general free radical scavenger as shown by its inhibition of o-dianisidine photo-oxidation. In this study, the effects of gliclazide and glibenclamide on free radicals were examined in vitro, using electron spin resonance spectroscopy. Superoxide radical (O₂^.−) generated from the hypoxanthine-xanthine oxidase system or hydroxyl radical (OH.) generated via the Fenton reaction were analyzed as spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Gliclazide scavenged O₂^.− and OH. in a dose-dependent manner whereas glibenclamide was without effect. These findings suggest that gliclazide is not only effective in reducing blood sugar, but may also be beneficial as a result of inhibition of lipid and protein denaturation, which is believed to lead to the development of diabetic complications.