Sodium salicylate was infused through a microdialysis probe placed in the striatum of anesthetized rats in order to assay the formation of hydroxyl radical (.OH) in the extracellular fluid in vivo. In addition to causing sustained dopamine release, intrastriatal infusion of the 2'-methyl analog of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (2'CH3-MPTP) increased the formation of 2,3-dihydroxybenzoic acid (2,3-DHBA), the nonenzymatic .OH adduct of salicylate in the brain dialysate. Inhibition of monoamine oxidase (MAO) by clorgyline and deprenyl completely blocked the formation of 2,3-DHBA and the sustained dopamine overflow induced by 2'-CH3-MPTP. The results indicate that the enhanced formation of cytotoxic .OH by 2'-CH3-MPTP is suppressed by MAO inhibitors. These data support the hypothesis that the protective effect of MAO inhibitors on the neurotoxicity induced by MPTP analogues may be due not only to the inhibition of MPTP metabolism by MAO but also the blockade of the formation of .OH free radicals. An enhanced generation of cytotoxic .OH free radicals in the striatum which in turn leads to oxidant damage may be relevant to the development of parkinsonism-like changes in animals produced by MPTP analogues.