When hippocampal synapses in culture are pharmacologically silenced for several days, synaptic strength increases. The structural correlate of this change in strength is an increase in the size of the synapses, with all synaptic components--active zone, postsynaptic density, and bouton--becoming larger. Further, the number of docked vesicles and the total number of vesicles per synapse increases, although the number of docked vesicles per area of active zone is unchanged. In parallel with these anatomical changes, the physiologically measured size of the readily releasable pool (RRP) and the release probability are increased. Ultrastructural analysis of individual synapses in which the RRP was previously measured reveals that, within measurement error, the same number of vesicles are docked as are estimated to be in the RRP.