Impairment of insulin signaling in the brain has been linked to neurodegenerative diseases. To test the hypothesis that neuronal insulin resistance contributes to defects in neuronal function, we have performed a detailed analysis of brain/neuron-specific insulin receptor knockout (NIRKO) mice. We find that NIRKO mice exhibit a complete loss of insulin-mediated activation of phosphatidylinositol 3-kinase and inhibition of neuronal apoptosis. In intact animals, this loss results in markedly reduced phosphorylation of Akt and GSK3β, leading to substantially increased phosphorylation of the microtubule-associated protein Tau, a hallmark of neurodegenerative diseases. Nevertheless, these animals exhibit no alteration in neuronal proliferation/survival, memory, or basal brain glucose metabolism. Thus, lack of insulin signaling in the brain may lead to changes in Akt and GSK3β activity and Tau hyperphosphorylation but must interact with other mechanisms for development of Alzheimer's disease.