Many high threshold, voltage-gated Ca²⁺ channels, including the dihydropyridine-sensitive class (L-type), inactivate in response not only to voltage, but also to entry of Ca²⁺. Despite the physiological importance of this Ca²⁺-sensitive inactivation, its molecular mechanism is understood only in broad outline. We now demonstrate that Ca²⁺-dependent inactivation transpires by a Ca²⁺-induced shift of channel gating to a low open probability mode, distinguished by a more than 100-fold reduction of entry rate to the open state. A gating mechanism that explains this shift quantitatively and enables successful separation of Ca²⁺- and voltage-sensitive forms of inactivation is deduced and tested. Finally, both calmodulin activation and channel (de)phosphorylation are excluded as significant signaling events underlying Ca ²⁺-induced mode shifts, leaving direct binding of Ca²⁺ to the channel as a likely chemical initiation event for inactivation.