The vulnerability of oligodendrocytes to excitatory amino acids may account for the pathology of white matter occurring following hypoxia/ischemia or autoimmune attack. Here, we examined the vulnerability of immature oligodendrocytes (positively labeled by galactocerobroside-C and not expressing myelin basic protein) from neonatal rat spinal cord to kainate, an agonist of excitatory amino acid receptors that induces long-lasting inward currents in immature oligodendrocytes. In particular, we studied whether kainate toxicity was linked to the endogenous production of nitric oxide. We found cultured oligodendrocytes to be highly sensitive to 24-48 h exposure to 0.5-1 mM kainate. The toxin induced striking morphological changes in oligodendrocytes, characterized by the disruption of the process network around the cell body and the growth of one or two long, thick and non-branched processes. A longer exposure to kainate resulted in massive death of oligodendrocytes, which was prevented by 6,7, dinitroquinoxaline-2,3-dione (DNQX) (30 micro M), the antagonist of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic/kainate receptors. Remarkably, we found that those oligodendrocytes displaying bipolar morphology following kainate exposure, also expressed the inducible form of nitric oxide synthase (iNOS) and nitrotyrosine immunoreactivity, suggesting that peroxynitrite could be formed by the reaction of nitric oxide with superoxide. Moreover, kainate toxicity was significantly prevented by addition of the NOS inhibitor nitro-L-arginine methyl ester (L-NAME), further suggesting that nitric oxide-derived oxidants contribute to excitotoxic mechanisms in immature oligodendrocytes.