BRIEF exposure of neuronally differentiated human NT2/D1 cells to hydrogen peroxide induced cell death by apoptosis with an ED50of 30 μM, whereas a 70-fold higher concentration was required to obtain an ED50effect in undifferentiated NT2/D1 neuronal precursor cells. This enhanced sensitivity in NT2/D1 neurons was correlated with an 8-fold lower level of intracellular glutathione. Pretreatment with N-acetyl-L-cysteine, an agent that is able to raise the levels of intracellular glutathione, promoted the survival of hydrogen peroxide-treated NT2/D1 neurons. Thus, the low glutathione level may contribute to the high sensitivity of NT2/D1 neurones to hydrogen peroxide-induced apoptosis. This study indicates that neuronal susceptibility to oxidative damage is developmentally regulated.