At presynaptic active zones (AZs), the frequently observed tethering of synaptic vesicles to an electron-dense cytomatrix represents a process of largely unknown functional significance. Here, we identified a hypomorphic allele,brp^nude, lacking merely the last 1% of the C-terminal amino acids (17 of 1740) of the active zone protein Bruchpilot. Inbrp^nude, electron-dense bodies were properly shaped, though entirely bare of synaptic vesicles. While basal glutamate release was unchanged, paired-pulse and sustained stimulation provoked depression. Furthermore, rapid recovery following sustained release was slowed. Our results causally link, with intramolecular precision, the tethering of vesicles at the AZ cytomatrix to synaptic depression.