Calcium entry into the postsynaptic neuron through N-methyl-D-aspartate-type glutamate receptors (NMDARs) triggers the induction of long-term potentiation (LTP), which is considered to contribute to synaptic plasticity and plays a critical role in behavioral learning. We report here that activin, a member of the transforming growth factor-beta (TGF-beta) superfamily, promotes phosphorylation of NMDARs and increases the Ca2+ influx through these receptors in primary cultured rat hippocampal neurons. This signal transduction occurs in a functional complex of activin receptors, NMDARs, and Src family tyrosine kinases, including Fyn, formed on a multimer of postsynaptic scaffolding postsynaptic density protein 95/Dlg/ZO-1 (PDZ), activin receptor interacting protein 1 (ARIP1). Activin-induced NMDAR activation persists for more than 24 h, which is complimentary to the activation time of NMDARs by brain-derived neurotrophic factor (BDNF). Our results suggest that activin is a unique and powerful potentiator for NMDAR-dependent signaling, which could be involved in the regulatory mechanisms of synaptic plasticity.