Deficits in learning and memory are prominent features of many mental disorders. Understanding molecular mechanisms responsible for learning and memory are key to the development of therapies to improve learning and memory in the treatment of mental illness. Recent studies lead to the exciting idea that newly generated neurons in the adult dentate gyrus (DG) of the hippocampus may play a role in neural plasticity including hippocampal-dependent memory formation. Adult neurogenesis occurs in the DG of the mammalian brains including adult human brain under physiological conditions and newly generated neurons functionally integrate to the DG. Many factors, including depression and other mental illness may adversely affect hippocampal adult neurogenesis. In contrast, treatment with anti-depressants and exercise, an effective means to treat depression, enhance adult neurogenesis in the DG. Consequently, elucidation of basic molecular mechanisms regulating adult hippocampal neurogenesis and generation of definitive evidence supporting a role for adult neurogenesis in hippocampal-dependent memory formation is a critical and timely issue for mental health research. Although many studies have assessed the role of adult neurogenesis in hippocampal-dependent learning and memory, the results have been inconsistent, making it still highly controversial whether adult neurogenesis contributes to hippocampus-dependent memory formation. This proposal will test the hypothesis that ERK5 signaling-mediated adult neurogenesis plays a critical role in some but not all forms of hippocampal- dependent learning and memory.