In previous studies with mice the oxygen radical generating neurotoxin tertiary butylhydroperoxide (t-BuOOH) was used to mimic the oxidative injury that has been implicated in neurodegenerative diseases. In addition, previous studies have shown that the poly (ADP-ribose) polymerase (PARP) inhibitor nicotinamide is able to prevent DNA fragmentation and apoptosis that is induced by t-BuOOH in mouse brain. However, the molecular mechanism(s) by which nicotinamide is able to protect human brain cells at the cellular level is not clear. Therefore in this study a cell culture model system with human cortical neuronal cells (HCN2 cells) has been employed where the molecular mechanism(s) of nicotinamide action, both in the presence and absence of t-BuOOH has been studied. Human cortical neurons (HCN2 cells) have been shown to differentiate to a neuron-like morphology. In this study, exposure of HCN2 cells to t-BuOOH resulted in altered morphology and disruption of neuronal differentiation leading to cell death. However, in neurons, which were treated with nicotinamide before being exposed to t-BuOOH, neuronal differentiation was preserved; morphological disruption was prevented and cell death was reduced significantly. Moreover, our studies indicate that nicotinamide is able to prevent the up-regulation of the pro-apoptotic proteins p53 and p21/WAF-1, and the down-regulation of the anti-apoptotic protein bcl-2 that is induced by t-BuOOH in HCN2 cells. Thus this study indicates that nicotinamide protects human brain cells from the toxic effects of free radical generating toxins by regulating the levels of various pro- and anti-apoptotic proteins.