Lead is a major environmental and occupational neurotoxicant. It has been shown that long-term exposure to a low level of lead impairs the development of brain. For example, it was reported that lead exposure during the childhood causes a learning difficulty and a memory deficit of children. Neurotoxic agents including the lead are believed to cause neuronal death in developing brain by two mechanisms: apoptosis and necrosis. However, the exact mechanism of neuronal death caused by lead exposure is still not known explicitly. In this study, we conducted a study to clarify a mechanism of hippocampal neuronal cell death caused by lead acetate. Hippocampal neurons were cultured for 14-16 days and treated with lead acetate of 1. 10, 100 1 microM concentrations for 12 hours. With the MTT(methyl tetrazolium test) kit, the viability of neuronal cells was measured. Next, in order to examine apoptosis caused by lead acetate, TUNEL (TdT-mediated d-UTP Nick End Labelling) assay was performed. It has been shown that lead acetate reduced the viability of neuronal cells in a dose dependent manner, especially at the concentration of 100 ~M lead acetate. TUNEL immunostain showed brownish signals in the nucleus of apoptotic cells. The proportions of apoptotic cells in the lead?acetate treated group were more higher than those in the controls and increased as lead acetate concentration increased. From above results, it may be concluded that lead in the hippocampal neuronal cells reduced cell viability and one of mechanisms in neuronal cell death by lead appears to be apoptosis.