Activity-driven delivery of AMPA receptors is proposed to mediate glutamatergic synaptic plasticity, both during development and learning. In hippocampal CA1 principal neurons, such trafficking is primarily mediated by the abundant GluR-A subunit. We now report a study of GluR-B_long, a C-terminal splice variant of the GluR-B subunit. GluR-B_long synaptic delivery is regulated by two forms of activity. Spontaneous synaptic activity-driven GluR-B_long transport maintains one-third of the steady-state AMPA receptor-mediated responses, while GluR-B_long delivery following the induction of LTP is responsible for approximately 50% of the resulting potentiation at the hippocampal CA3 to CA1 synapses at the time of GluR-B_long peak expression—the second postnatal week. Trafficking of GluR-B_long-containing receptors thus mediates a GluR-A-independent form of glutamatergic synaptic plasticity in the juvenile hippocampus.