We recently reported that para-nonylphenol, an environmental chemical, induced hydroxyl radical (*OH) formation in rat striatum. In this study we examined the antioxidant effects of angiotensin-converting enzyme inhibitors (captopril or enalaprilat) on para-nonylphenol (nonylphenol) and 1-methyl-4-phenylpyridinium ion (MPP(+))-induced hydroxyl radical (*OH) formation and dopamine (DA) efflux in extracellular fluid of rat striatum, using a microdialysis technique. para-Nonylphenol clearly enhanced *OH formation and DA efflux induced by MPP(+). When captopril or enalaprilat was infused in nonylphenol and MPP(+)-treated rats, DA efflux and OH formation significantly decreased, as compared with that in the nonylphenol and MPP(+)-treated control. We compared the ability of non-SH-containing enalaprilat with a SH-containing captopril to scavenge OH and DA efflux. Both inhibitors were able to scavenge *OH and DA efflux induced by para-nonylphenol and MPP(+). The results suggest that angiotensin-converting enzyme inhibitors may protect against nonylphenol and MPP(+)-induced *OH formation via suppressing DA efflux in the rat striatum.