Glycogen synthase kinase-3 (GSK-3) is thought to play an important role in the hyperphosphorylation of tau, and possibly other proteins, in Alzheimer's disease (AD). However, the effects of GSK-3 on neuronal metabolism are still largely unknown. Here we describe that a low concentration of lithium, which can partially inhibit endogenous GSK-3, favored the extension of neurites from developing neurons, whereas a high concentration of lithium impaired neurite growth. Furthermore, the overexpression of exogenous active GSK-3 in neurons by infection with a defective herpesviral vector blocked neurite growth, which was not affected by either expression of inactive GSK-3 or just the herpesviral vector infection. Neurite extension was restored when neurons overexpressing exogenous active GSK-3 were incubated with lithium. These results are consistent with a role for GSK-3 in the regulation of cytoskeletal dynamics during neurite growth. Accordingly, up-regulation of GSK-3 may contribute to cytoskeletal pathology within neurites in AD.