Neurotransmitter release at many central synapses is initiated by an influx of calcium ions through P/Q-type calcium channels(1,2), which are densely localized in nerve terminals(3). Because neurotransmitter release is proportional to the fourth power of calcium concentration(4,5), regulation of its entry can profoundly influence neurotransmission. N- and P/Q-type calcium channels are inhibited by G proteins(6,7), and recent evidence indicates feedback regulation of P/Q-type channels by calcium(8). Although calcium-dependent inactivation of L-type channels is well documented(9-11), little is known about how calcium modulates P/Q-type channels. Here we report a calcium-dependent interaction between calmodulin and a novel site in the carboxy-terminal domain of the cu,A subunit of P/Q-type channels. In the presence of low concentrations of intracellular calcium chelators, calcium influx through P/Q-type channels enhances channel inactivation, increases recovery from inactivation and produces a long-lasting facilitation of the calcium current. These effects are prevented by overexpression of a calmodulin-binding inhibitor peptide and by deletion of the calmodulin-binding domain. Our results reveal an unexpected association of Ca2+/calmodulin with P/Q-type calcium channels that may contribute to calcium-dependent synaptic plasticity.