Lithium (Li⁺) inhibits glycogen synthase kinase-3 (GSK-3) activity and mimics Wingless signaling in intact cells. This study shows that Li⁺ specifically inhibits GSK-3 family members in vitro and in intact cells, and that it can mimic Wingless signaling. The findings suggest that Li⁺ acts as a specific inhibitor of GSK-3, which is a highly conserved serine/threonine kinase involved in cell fate determination and signaling pathways. In Drosophila, GSK-3 is encoded by Zw3sgg, a gene involved in embryogenesis that acts downstream of Wingless. In higher eukaryotes, GSK-3 is involved in signal transduction pathways downstream of phosphoinositide 3-kinase and mitogen-activated protein kinases. Li⁺ inhibits GSK-3 activity in vitro, and this effect is reversible. Li⁺ treatment inhibits GSK-3-dependent phosphorylation of the microtubule-associated protein Tau. In Drosophila S2 cells and rat PC12 cells, Li⁺ treatment induces accumulation of cytoplasmic Armadillo/β-catenin, demonstrating that Li⁺ can mimic Wingless signaling in intact cells. These results suggest that Li⁺ mimics Wingless signaling by inhibiting GSK-3, which is consistent with its effects on development and differentiation. The study also shows that Li⁺ inhibits GSK-3-dependent phosphorylation of Tau in intact cells. Li⁺ treatment reduces AT8 immunoreactivity, which is a marker of Tau phosphorylation, without altering Tau levels. These findings support the hypothesis that Li⁺ inhibits GSK-3 and that this inhibition contributes to its effects on development and differentiation. The results indicate that Li⁺ acts as a specific inhibitor of GSK-3 and that it mimics Wingless signaling in intact cells. This suggests that GSK-3 is an important physiological target of Li⁺ action.Lithium (Li⁺) inhibits glycogen synthase kinase-3 (GSK-3) activity and mimics Wingless signaling in intact cells. This study shows that Li⁺ specifically inhibits GSK-3 family members in vitro and in intact cells, and that it can mimic Wingless signaling. The findings suggest that Li⁺ acts as a specific inhibitor of GSK-3, which is a highly conserved serine/threonine kinase involved in cell fate determination and signaling pathways. In Drosophila, GSK-3 is encoded by Zw3sgg, a gene involved in embryogenesis that acts downstream of Wingless. In higher eukaryotes, GSK-3 is involved in signal transduction pathways downstream of phosphoinositide 3-kinase and mitogen-activated protein kinases. Li⁺ inhibits GSK-3 activity in vitro, and this effect is reversible. Li⁺ treatment inhibits GSK-3-dependent phosphorylation of the microtubule-associated protein Tau. In Drosophila S2 cells and rat PC12 cells, Li⁺ treatment induces accumulation of cytoplasmic Armadillo/β-catenin, demonstrating that Li⁺ can mimic Wingless signaling in intact cells. These results suggest that Li⁺ mimics Wingless signaling by inhibiting GSK-3, which is consistent with its effects on development and differentiation. The study also shows that Li⁺ inhibits GSK-3-dependent phosphorylation of Tau in intact cells. Li⁺ treatment reduces AT8 immunoreactivity, which is a marker of Tau phosphorylation, without altering Tau levels. These findings support the hypothesis that Li⁺ inhibits GSK-3 and that this inhibition contributes to its effects on development and differentiation. The results indicate that Li⁺ acts as a specific inhibitor of GSK-3 and that it mimics Wingless signaling in intact cells. This suggests that GSK-3 is an important physiological target of Li⁺ action.