We showed previously that dopamine (DA) release in dorsal striatum is inhibited by endogenously generated hydrogen peroxide (H(2)O(2)). Here, we examined whether endogenous H(2)O(2) can also modulate somatodendritic DA release in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA), with companion measurements in DA terminal regions. Evoked DA release was monitored in brain slices using carbon-fiber microelectrodes with fast-scan cyclic voltammetry. Exogenous H(2)O(2) decreased DA release by 50-60% in SNc and VTA but only by 35% in nucleus accumbens. Whether endogenous H(2)O(2) also modulated somatodendritic release was examined using the glutathione peroxidase inhibitor, mercaptosuccinate (MCS), which should increase stimulation-evoked H(2)O(2) levels. In the presence of MCS, DA release was suppressed by 30-40% in SNc as well as in dorsal striatum and nucleus accumbens. In striking contrast, DA release in the VTA was unaffected by MCS. These data are consistent with stronger H(2)O(2) regulation or lower H(2)O(2) generation in VTA than in the other regions. Importantly, oxidative stress has been linked causally to Parkinson's disease, in which DA cells in SNc degenerate, but VTA cells are spared. The present data suggest that differences in oxidant regulation or generation between SNc and VTA could contribute to this.