Glycine and GABA_A (γ-aminobutyric acid A) receptors are inhibitory neurotransmitter-gated Cl^- channels localized in postsynaptic membranes. In some cases, GABA_A receptors are also found presynaptically, but they retain their inhibitory effect as their activation reduces excitatory transmitter release^,,,. Here we report evidence for presynaptic ionotropic glycine receptors, using pre- and postsynaptic recordings of a calyceal synapse in the medial nucleus of the trapezoid body (MNTB). Unlike the classical action of glycine, presynaptic glycine receptors triggered a weakly depolarizing Cl^- current in the nerve terminal. The depolarization enhanced transmitter release by activating Ca²⁺ channels and increasing resting intraterminal Ca²⁺ concentrations. Repetitive activation of glycinergic synapses on MNTB neurons also enhanced glutamatergic synaptic currents, indicating that presynaptic glycine receptors are activated by glycine spillover. These results reveal a novel site of action of the transmitter glycine, and indicate that under certain conditions presynaptic Cl^- channels may increase transmitter release.