The distance between Ca²⁺ channels and synaptic vesicles determines the time course and probability of release. We have examined the coupling between Ca²⁺ channels and synaptic vesicles in the GABAergic synapses between interneurons [dentate gyrus basket cells (BCs)] and principal neurons [dentate gyrus granule cells (GCs)]. Dual whole-cell patch-clamp recordings were made from synaptically connected BC-GC pairs in acute hippocampal slices from 18- to 21-day-old rats. The fast Ca²⁺ buffer BAPTA was loaded into the presynaptic BC by perfusion of the recording pipette using a pressure-vacuum pump. BAPTA reduced unitary inhibitory postsynaptic current (IPSC) amplitudes in a concentration-dependent manner (IC₅₀= 2.8 mM, measured 50–65 min after application onset; 8 pairs). The effect of BAPTA on multiple pulse depression (MPD) was examined in trains of 10 IPSCs evoked at 50 Hz. 3 mM BAPTA did not affect the MPD, whereas 10 mM BAPTA increased the ratio IPSC₁₀ / IPSC₁ from 16 to 39%. Based on the IC₅₀ value, we estimate that the average distance between Ca²⁺ channels and synaptic vesicles is approximately [lambda] = [D_Ca / (k_on * IC₅₀)]^1/2 [cong] 20 nm, where D_Ca is the diffusion coefficient for Ca²⁺, 220 mm² s ^-1 and k_on is the Ca²⁺ binding rate of BAPTA, 4 10⁸ M^­1 s ^­1. Thus, unlike other central synapses, BC-GC synapses show a tight, nanometer range coupling between Ca²⁺ channels and synaptic vesicles.