Lithium modulates glycogen synthase kinase 3β (GSK-3β), a kinase involved in Alzheimer disease-related tau pathology. To investigate mechanisms of aging and the potential therapy of lithium in neurodegenerative disease, we treated senescence-accelerated mouse (SAM)P8 mice, a murine model of senescence, and mice of the control SAMR1 strain with lithium. The treatment reduced hippocampal caspase 3 and calpain activation, indicating that it provides neuroprotection. Lithium also reduced both the levels and activity of GSK-3β and the activity of cyclin-dependent kinase 5 and reduced hyperphosphorylation of 3 different phosphoepitopes of tau: Ser199, Ser212, and Ser396. In lithium-treated primary cultures of SAMP8 and SAMR1 cerebellar neurons, there was a marked reduction in protease activity mediated by calpain and caspase 3. Both lithium and SB415286, a specific inhibitor of GSK-3β, reduced apoptosis in vitro. Taken together, these in vivo and in vitro findings of lithium-mediated reductions in GSK-3β and cyclin-dependent kinase 5 activities, tau phosphorylation, apoptotic activity, and cell death provide a strong rationale for the use of lithium as a potential treatment in neurodegenerative diseases.