AbstractFragile X syndrome (FXS), the most common heritable form of mental retardation, is characterized by synaptic dysfunction. Synaptic transmission depends critically on presynaptic calcium entry via voltage-gated calcium (Ca_V) channels. Here we show that the functional expression of neuronal N-type Ca_V channels (Ca_V2.2) is regulated by fragile X mental retardation protein (FMRP). We find that FMRP knockdown in dorsal root ganglion neurons increases Ca_V channel density in somata and in presynaptic terminals. We then show that FMRP controls Ca_V2.2 surface expression by targeting the channels to the proteasome for degradation. The interaction between FMRP and Ca_V2.2 occurs between the carboxy-terminal domain of FMRP and domains of Ca_V2.2 known to interact with the neurotransmitter release machinery. Finally, we show that FMRP controls synaptic exocytosis via Ca_V2.2 channels. Our data indicate that FMRP is a potent regulator of presynaptic activity, and its loss is likely to contribute to synaptic dysfunction in FXS.