Intracellular glutathione (GSH) levels determine whether nitric oxide (NO) is neurotrophic for dopamine neurons or triggers a cell death cascade in primary midbrain cultures. We have investigated herein the role of the extracellular-signal regulated protein kinase (ERK) 1/2 pathway in this GSH switching effect. The short-lived NO donor DEA/NO induces a transient activation of ERK-1/2 that totally disappears 2 h after NO administration. The depletion of GSH increases and the supplementation of GSH suppresses ERK-1/2 activation in response to NO treatment. More interestingly, GSH depletion changes the kinetic of phosphorylation leading to a second prolonged phase of ERK-1/2 activation from 2 to 16 h after NO addition. This change of kinetic is ultimately responsible for NO toxicity under GSH-depleted conditions, because selective blockade of the second and persistent phase of activation prevents cell death. In addition, the only transient ERK activation, induced by NO under normal GSH conditions, did not cause ERK-dependent cell death. Immunocytochemical colocalization studies demonstrate that ERK activation takes place exclusively in glial cells, mainly in astrocytes and less frequently in oligodendrocytes and glial progenitors. Furthermore, glial cell elimination or inactivation in the culture, by gliotoxic drugs, abrogates NO-induced ERK activation. Our results indicate that neurotrophism of NO switches into neurotoxicity after GSH depletion due to persistent activation of the ERK-1/2 signaling pathway in glial cells. The implication of these results in pathological conditions like Parkinson's disease, where GSH depletion and NO overproduction have been documented, are discussed.