OBJECTIVES: To review the understanding about co-localisation of amino acid transmitters in the brain. RESULTS: The idea that neurons release the same transmitter at all their synapses is associated with Henry Dale and formulated as Dale's principle by John Eccles. This idea has been modified during the last years based on several studies showing that transmitters can co-exist at the same synapse. First, a large body of evidence was presented showing that a classical transmitter can be co-localized with different types of neuropeptides. Then, several studies showed that a synapse could co-release two classical transmitters. CONCLUSION: This review presents and discusses data from our laboratory showing co-release of glutamate/gamma-aminobutyric acid (GABA), aspartate/glutamate and aspartate/GABA from different types of hippocampal synapses.