NEURONAL injury resulting from acute brain insults and some neurodegenerative diseases implicates N-methyl-D-aspartate (NMDA) glutamate receptors^1–4. The fact that antioxidants reduce some types of brain damage suggests that oxygen radicals may have a role^5–7. It has been shown that mutations in Cu/Zn-superoxide dismutase (SOD), an enzyme catalysing superoxide (O^·-₂) detoxification in the cell, are linked to a familial form of amyotrophic lateral sclerosis (ALS)⁴. Here we report that O^·-₂ is produced upon NMDA receptor stimulation in cultured cerebellar granule cells. Electron paramagnetic resonance was used to assess O^·-₂ production that was due in part to the release of arachidonic acid. Activation of kainic acid receptors, or voltage-sensitive Ca²⁺ channels, did not produce detectable O^·-₂. We also find that the nitrone DMPO (5,5-dimethyl pyrroline 1-oxide), used as a spin trap, is more efficient than the nitric oxide synthase inhibitor, L-N^G-nitroarginine, in reducing NMDA-induced neuronal death in these cultures.