Many glutamate synapses early in development are so called AMPA silent, i.e. they signal only via NMDA receptors. We have proposed that this is not because the glutamate synapse is born AMPA silent. Instead it is born "AMPA labile" meaning that AMPA signaling is easily lost whenever the synapse is activated and is slowly regained when the synapse is inactive. This AMPA lability is found among about half of the synapses onto developing principal neurons and interneurons as well as among synapses on mature interneurons in the hippocampus. AMPA labile synapses become stabilized by correlated pre-and postsynaptic activity (LTP-inducing activity). This stabilization of AMPA labile synapses accounts for all "LTP" during the first two postnatal weeks. In other words, at this developmental stage LTP-inducing activity does not potentiate transmission, but merely stabilizes it. Our results thus suggest three different states of AMPA signaling at developing hippocampal glutamate synapses; "labile", "silent" and "stable".