•  Inhibition mediated by GABA and glycine is essential for controlling a balance of excitation and inhibition in the spinal cord. •  Although these transmitters are known to be co‐released from the same synaptic vesicles, the mechanisms that control the packaging of GABA + glycine into synaptic vesicles have not been fully characterized. •  In this study, using paired whole‐cell recording, we found that raised extracellular glutamate levels increased the amplitude of GABAergic IPSCs by enhancing glutamate uptake but reduced glycine release. •  High‐frequency trains of stimulation decreased glycinergic IPSCs more than GABAergic IPSCs at GABA/glycine mixed synapses, and repetitive stimulation occasionally failed to evoke glycinergic but not GABAergic IPSCs. •  The present results suggest that the use of GABA as a transmitter at GABA/glycine mixed synapses may afford protection against pathophysiological hyperexcitability associated with increased extracellular glutamate concentration. Abstract  Fast inhibitory neurotransmission in the central nervous system is mediated by γ‐aminobutyric acid (GABA) and glycine, which are accumulated into synaptic vesicles by a common vesicular inhibitory amino acid transporter (VIAAT) and are then co‐released. However, the mechanisms that control the packaging of GABA + glycine into synaptic vesicles are not fully understood. In this study, we demonstrate the dynamic control of the GABA–glycine co‐transmission by the neuronal glutamate transporter, using paired whole‐cell patch recording from monosynaptically coupled cultured spinal cord neurons derived from VIAAT‐Venus transgenic rats. Short step depolarization of presynaptic neurons evoked unitary (cell‐to‐cell) inhibitory postsynaptic currents (IPSCs). Under normal conditions, the fractional contribution of postsynaptic GABA or glycine receptors to the unitary IPSCs did not change during a 1 h recording. Intracellular loading of GABA or glycine via a patch pipette enhanced the respective components of inhibitory transmission, indicating the importance of the cytoplasmic concentration of inhibitory transmitters. Raised extracellular glutamate levels increased the amplitude of GABAergic IPSCs but reduced glycine release by enhancing glutamate uptake. Similar effects were observed when presynaptic neurons were intracellularly perfused with glutamate. Interestingly, high‐frequency trains of stimulation decreased glycinergic IPSCs more than GABAergic IPSCs, and repetitive stimulation occasionally failed to evoke glycinergic but not GABAergic IPSCs. The present results suggest that the enhancement of GABA release by glutamate uptake may be advantageous for rapid vesicular refilling of the inhibitory transmitter at mixed GABA/glycinergic synapses and thus may help prevent hyperexcitability.