In order to evaluate the cytotoxicity of methylmercuric chloride(MMC) in cultured spinal motor neurons of neonatal mouse, cell viability was measured by MTT assay in spinal motor neurons treated with 1-30 μM MMC for 48 hours. And also, the protective effect of Radix Polygoni Multiflori(RPM) was examined by cell viability in these cultures. Cell viability was significantly decreased in dose-dependent manner after cultured cells were exposured to 20 μM MMC for 48 hours. Protective effect of RPM on MMC-mediated toxicity was very effective in these cultures. From above the results, it suggests that MMC has toxic effect in cultured mouse spinal motor neurons and herb extract such as RPM is very effective in blocking the neurotoxicity induced by MMC.