The enhancement of hippocampal neurogenesis by ω-3 polyunsaturated fatty acids is an important tool for treating neurodegenerative diseases, such as Alzheimer's disease, and to enhance learning and memory. The exact mechanisms of action, however, remain poorly understood. We have previously reported that docosahexaenoic acid (DHA) decreases mRNA levels of Hes1, an inhibitor of neuronal differentiation, and increases NeuroD and Map2 mRNA, as well as the number of TuJ1-positive (a neuronal marker) cells, indicating that DHA induces neuronal differentiation. Eicosapentaenoic acid (EPA) increases mRNA levels of Hes1, Hes6, an inhibitor of Hes1, NeuroD, and Map2, as well as TuJ1-positive cells, indicating that EPA also induces neuronal differentiation. However, treatment with arachidonic acid (ARA) decreases Hes1 mRNA, but does not affect mRNA expression of NeuroD and Map2. Furthermore, ARA does not affect the number of TuJ1-positive cells. DHA and EPA, but not ARA, increase mRNA levels of p21 and p27, cyclin-dependent kinase inhibitors, indicating that DHA and EPA induce cell cycle arrest. These results suggest that EPA could be involved in neuronal differentiation by mechanisms that are different to DHA. We also have reported that N-docosahexaenoylethanolamine (synaptamide), an endogenous DHA metabolite with an endocannabinoid-like structure, promotes neuronal differentiation of neural stem cells at a lower dose than DHA and that its action takes place via protein kinase A/cAMP response element binding protein activation. These results suggest that the transduction of intracellular signaling is involved in ω-3 polyunsaturated fatty acid-induced neurogenesis. However, the receptors or binding proteins are involved this mechanism, as well as other metabolites, remain to be shown.