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Presynaptic Ca²⁺ signaling plays a crucial role in short-term plasticity of synaptic transmission. Here, we studied the role of mobile endogenous presynaptic Ca²⁺ buffer(s) in modulating paired-pulse facilitation at a large excitatory nerve terminal in the auditory brainstem, the calyx of Held. To do so, we assessed the effect of presynaptic whole-cell recording, which should lead to the diffusional loss of endogenous mobile Ca²⁺ buffers, on paired-pulse facilitation and on intracellular Ca²⁺ concentration ([Ca²⁺]i) transients evoked by action potentials. In unperturbed calyces briefly preloaded with the Ca²⁺ indicator fura-6F, the [Ca²⁺]i transient decayed surprisingly fast (θfast, ∼30 ms). Presynaptic whole-cell recordings made without additional Ca²⁺ buffers slowed the decay kinetics of [Ca²⁺]i and paired-pulse facilitation (twofold to threefold), but the amplitude of the [Ca²⁺]i transient was changed only marginally. The fast [Ca²⁺]i decay was restored by adding the slow Ca²⁺ buffer EGTA (50–100 µM) or parvalbumin (100 µM), a Ca²⁺-binding protein with slow Ca²⁺-binding kinetics, to the presynaptic pipette solution. In contrast, the fast Ca²⁺ buffer fura-2 strongly reduced the amplitude of the [Ca²⁺]i transient and slowed its decay, suggesting that the mobile endogenous buffer in calyces of Held has slow, rather than fast, binding kinetics. In parvalbumin knock-out mice, the decay of [Ca²⁺]i and facilitation was slowed approximately twofold compared with wild-type mice, similar to what is observed during whole-cell recordings in rat calyces of Held. Thus, in young calyces of Held, a mobile Ca²⁺ buffer with slow binding kinetics, primarily represented by parvalbumin, accelerates the decay of spatially averaged [Ca²⁺]i and paired-pulse facilitation.