Methylmercury (MeHg) is recognized as a significant environmental hazard, particularly to the development of the nervous system. To improve our understanding of risk associated with MeHg exposure, and elucidate the mechanism for developmental toxicity of MeHg, in this dissertation, molecular mechanism underlying cell cycle arrest by MeHg and the role of intracellular glutathione (GSH) in MeHg-induced toxicity were examined using primary rodent embryonic neuronal cell (CNS) and limb bud (LB) cultures. Gadd45, Gadd153 and p21 are cell cycle regulatory genes involved in the control of the G1 and G2/M arrest. In the present study, a differential endogenous basal expression of these genes was observed during CNS and LB differentiation in culture. However, both CNS and LB cells responded to MeHg exposure with a marked increase in Gadd45 and Gadd153 and a lesser degree of p21 mRNA expression. The result represents the first evidence that MeHg exposure can alter the expression of genes which control cell cycle progression, a fundamental event in embryonic development. Although these results have not been confirmed in embryonic cells in situ, exposure of the adult CNS in vivo to a chronic low dose of MeHg activates the expression of p21. In addition, the data indicate that both necrotic and apoptotic cell death are associated with the activation of cell cycle arrest genes following MeHg exposure. Since induction of the cell cycle arrest does not fully account for the cytotoxicity of MeHg, intracellular GSH, an important component of the cellular defense mechanism against oxidative stress was also examined. However, the results suggest that changes in intracellular GSH levels alone can not account for the sensitivity of the embryonic CNS to MeHg exposure. Taken together, the data provide a basis for future investigations on the role of cell cycle regulatory genes, cell death and GSH in MeHg toxicity in vivo. Continued examination of molecular events underlying cell cycle inhibition by MeHg would further advance our understanding of the mechanism associated with MeHg toxicity in developing CNS.