Activity-dependent alteration in synaptic strength is a fundamental property of the vertebrate central nervous system and is thought to underlie learning and memory. The most extensively studied model of activity-dependent synaptic plasticity is long-term potentiation (LTP) of glutamate-responsive (glutamatergic) synapses, a widespread phenomenon involving multiple mechanisms. The best characterized form of LTP occurs in the CA1 region of the hippocampus, in which LTP is initiated by transient activation of NMDA (N-methyl-D-aspartate) receptors and is expressed as a persistent increase in synaptic transmission through AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate) receptors. This increase is due, at least in part, to a postsynaptic modification of AMPA-receptor function; this modification could be caused by an increase in the number of receptors, their open probability, their kinetics or their single-channel conductance. Here we show that the induction of LTP in the CA1 region of the hippocampus is often associated with an increase in single-channel conductance of AMPA receptors. This shows that elementary channel properties can be rapidly modified by synaptic activity and provides an insight into one molecular mechanism by which glutamatergic synapses can alter their strength.