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Summary: Voltage-dependent Ca2+ channels (VGCC) represent the principal source of Ca2+ ions driving evoked neurotransmitter release at presynaptic boutons. In mammals, presynaptic Ca2+ influx is mediated mainly via P/Q-type and N-type VGCC, which differ in their properties. Changes in their relative contributions tune neurotransmission both during development and in Hebbian plasticity. However, whether this represents a functional motif also present in other forms of activity-dependent regulation is unknown. Here, we study the role of VGCC in homeostatic plasticity (HSP) in mammalian hippocampal neurons using optical techniques. We find that changes in evoked Ca2+ currents specifically through P/Q-type, but not N-type, VGCC mediate bidirectional homeostatic regulation of both neurotransmitter release efficacy and the size of the major synaptic vesicle pools. Selective dependence of HSP on P/Q-type VGCC in mammalian terminals has important implications for phenotypes associated with P/Q-type channelopathies, including migraine and epilepsy. : Jeans at al. show that both basal neurotransmission and synaptic vesicle pool sizes are specifically regulated by the presynaptic P/Q-type voltage-gated Ca2+ channel during HSP at mammalian hippocampal synapses. This may shed light on mechanisms underlying phenotypes associated with P/Q-type channelopathies, including migraine and epilepsy. Keywords: synapse, homeostatic plasticity, voltage-gated calcium channel, neurotransmitter release, pHluorin, P/Q-type channelopathy