Nigral cell degeneration induced by 1-methyl-4-phenyl-1,2,3,6-tertrahydropyridine (MPTP) or its metabolite 1-methy1-4-phenyl pyridinium (MMP+) may involve toxicity induced by nitric oxide. In the present study a microdialysis procedure incorporating salicylate hydroxylation was used to measure striatal hydroxyl radical production through the formation of 2,3-dihydroxybenzoic acid (2,3-DHBA). MPP+ (5–20 mM for 20 min) increased 2,3-DHBA formation in the rat striatum in a concentration-dependent manner with a concomitant increase in dopamine release and decrease in 3,4-dihydroxyphenyl acetic acid (DOPAC) formation. Inhibition of NO synthesis following NG-nitro-L-arginine methyl ester (L-NAME; 1 mM) and 7-nitroindazole monosodium salt (7-NINA; 1 mM), but not NG-nitro-D-arginine methyl ester (D-NAME; 1 mM) attenuated the MPP+-induced increase in hydroxyl radical formation. However, neither L-NAME nor 7-NINA had any effect on the MPP+-induced increase in dopamine efflux measured in vivo by microdialysis or in vitro using superfused striatal slices, although nomifensine (10 μM) abolished the MPP+-evoked dopamine efflux in vitro. These data suggest that NO formation is necessary for the production of hydroxyl radical following MPP+ treatment, but is not involved in the MPP+-evoked dopamine release.