Factors controlling proliferation and differentiation are crucial in advancement of neural cell-based experimental neurodegenerative therapies. In this regard, nicotinamide has been shown to determine the fate of neural cells, enhance neuralization, and influence DNA repair and apoptosis. This study investigated whether the biologically active vitamin B3 metabolite, nicotinamide, could direct the differentiation of mouse embryonic stem cells, cultured as monolayers, into neurons at either early or late stages of development. Interestingly, we observed a dose-responsive increase in the percentage of neurons when nicotinamide was added at early stages to the cells undergoing differentiation (days 0-7). Nicotinamide (10 mM) had a significant effect on neuronal differentiation, increasing the βIII-tubulin-positive neuronal population and concomitantly decreasing the total number of cells in culture, measured by quantification of 4',6-diamidino-2-phenylindole (DAPI)-positive cells. Nicotinamide added between days 7 and 14 had no effect on neuronal induction. High levels of nicotinamide (20 mM) induced cytotoxicity and cell death. Current work is focusing on elucidating the mechanism(s) mediating neural specification by nicotinamide--that is, induction of cell-cycle exit and/or selective apoptosis in non-neural populations. Preliminary data suggest a reduction in the proportion of proliferating cells in nicotinamide-treated cultures--that is, nicotinamide enhances cell-cycle exit, thereby promoting neuronal differentiation. Future work will focus on evaluating the effect of nicotinamide on the differentiation of midbrain dopamine neurons, towards a therapy for Parkinson's disease.