SYNAPTIC transmission in the hippocampus is rather unreliable, with many presynaptic action potentials failing to release neurotransmitter(1-4). How is this unreliability affected by the alterations in synaptic strength seen in long-term potentiation (LTP)(5) and long-term depression(6,7) (LTD)? We find that LTP increases synaptic reliability, and LTD decreases it, both without a change in the size of those postsynaptic currents that do occur. Thus LTD is a functional inverse of LTP.