In order to elucidate the mechanism of cytotoxic effect of oxygen radicals on cultured mouse spinal motor neurons, the neurotoxicity induced by hydrogen peroxide(H₂O₂) was evaluated by MTT assay. The neuroprotective effect of Rhizoma Gastrodiae(RG) against H₂O₂-mediated neurotoxicity was also examined in these cultures by SRB assay. The results were as follows : The value of lethal concentration 50(LC50) of H₂O₂ was estimated at a concentration of 30 uM in these cultures. Cell viability of cultured mouse spinal motor neurons was remarkably decreased by H₂O₂-induced neurotoxicity in a dose- and time-dependent manner. RG was remarkably effective in blocking the neurotoxicity induced by H₂O₂ at a concentration of 120 μM as determined by SRB assay. From above the results, it is suggested that H₂O₂ induce neurotoxicity, and the selective herbal extracted RG was very effective in blocking H₂O₂-mediated neurotoxicity on cultured mouse spinal motor neurons.