Many activity-dependent changes in synaptic efficacy occur through elevations in postsynaptic calcium triggered by glutamate receptor activation. Here, the postsynaptic, neuron-specific microtubule-associated protein MAP2 is identified as a target of bidirectional calcium-dependent signaling pathways activated by glutamate. Glutamate produced a biphasic change in MAP2: a rapid, transient increase in phosphorylation mediated by metabotropic receptors and attenuated by inhibitors of calcium/calmodulin-dependent protein kinases and protein kinase C, followed by a persistent dephosphorylation of MAP2 mediated by NMDA receptors and activation of the calcium/calmodulin-dependent protein phosphatase 2B (calcineurin). Thus, a single transmembrane signal, glutamate, and the increased intracellular calcium it evokes can have opposing actions on a postsynaptic target phosphoprotein. The phosphorylation state of MAP2 determines its interaction with microtubules and actin filaments, suggesting that glutamatergic regulation of MAP2 phosphorylation may transduce neural activity into modifications in dendritic structure.