In order to elucidate the mechanism between cytotoxicity of methhylmercuric chloride(MMC) and oxygen radicals in cultured osteoblasts of neonatal mouse, cell viability was measured by MTT assay in osteoblasts treated with 1～50 μM MMC for 30 hours. And also, the protective effect of N-methyl D-aspartate(NMDA) receptor antagonist, D-2-amino-5-phosphovaleric acid(APV) was examined by cell viability in these cultures. Cell viability was significantly decreased in dose dependently after exposure of 30 μM MMC to cultured osteoblasts for 30 hours. Protective effect of APV against MMC-mediated toxicity was very effective in these cultures. From above the results, it suggests that MMC is toxic in cultured mouse osteoblasts and NMDA receptor antagonist such as APV is effective in blocking the osteotoxicity induced by MMC.