Long-term enhancement of synaptic efficacy in the hippocampus is an important model for studying the cellular mechanisms of neuronal plasticity, circuit reorganization, and even learning and memory(1). Although these long-lasting functional changes are easy to induce, it has been very difficult to demonstrate that they are accompanied or even caused by morphological changes on the subcellular level Here we combined a local superfusion technique(2,3) with two-photon imaging(4), which allowed us to scrutinize specific regions of the postsynaptic dendrite where we knew that the synaptic changes had to occur. We show that after induction of long-lasting (but not short-lasting) functional enhancement of synapses in area CAI, new spines appear on the postsynaptic dendrite, whereas in control regions on the same dendrite or in slices where long-term potentiation was blocked, no significant spine growth occurred.