The chick ciliary ganglion calyx-type nerve terminal was used to examine voltage-sensitive inactivation of presynaptic N-type Ca2+ channels and to test if this inactivation is modulated by the transmitter release-associated protein syntaxin I. We tested the role of this protein with botulinum toxin C1 (BtC1) which cleaves syntaxin I close to its membrane anchor. The presynaptic Ca2+ current inactivated as two distinct populations with ∼75% inactivating at a depolarized potential, V1/2∼−15 mV, with the remainder inactivating at ∼−75 mV. BtC1 had no detectable effect on the latter component but resulted in a ∼7 mV positive shift in the V1/2 of the −15 mV inactivating component. These results confirm that the bulk of presynaptic N-type Ca2+ channels are in general resistant to voltage dependent inactivation and provide the first direct evidence that the physiological properties of presynaptic nerve terminal Ca2+ channels are subject to modulation by release site-associated proteins.