Addition of 1 microM-1 mM methadone to the bathing medium produced a concentration-dependent reduction in the neurotoxicity of exogenously applied N-methyl-D-aspartate (NMDA) in murine cortical cell culture (EC50 about 100 microM); the reduction persisted at intense NMDA exposure, consistent with non-competitive inhibition. Methadone also protected against exposure to quinolinate but not quisqualate or kainate. Concentrations (100 microM-3 mM) of several other opioids - morphine, fentanyl, codeine, meperidine, dextropropoxyphene, and naltrexone - were additionally found to produce concentration-dependent reductions in NMDA neurotoxicity. This novel neuron-protective effect of opioids was not mediated by conventional opioid receptors: the non-opioid enantiomer of methadone and morphine exhibited a potency equal to or greater than that of the opioid enantiomer, and 1 mM naloxone did not act as an antagonist. The possibility that opioids, or especially non-opioid enantiomers of opioids, might provide a useful therapeutic approach in diseases states involving NMDA receptor-mediated neurotoxicity, warrants further study.