Tonic release of glutamate into the extracellular space of the hippocampus and striatum is non-vesicular, and has been attributed largely to a cystine–glutamate exchanger which is blockable by the glutamate analogue (S)-4-carboxyphenylglycine (CPG). Tonic glutamate release may be functionally important: modulation of this release in the striatum has been suggested to underlie relapse in the use of cocaine. We monitored tonic glutamate release in area CA1 of hippocampal slices by measuring the glutamate receptor-mediated current evoked in pyramidal cells on block of Na+-dependent glutamate uptake with dl-threo-β-benzyloxyaspartate (TBOA). Superfused cystine increased tonic glutamate release, and this increase was blocked by CPG, but CPG did not affect tonic glutamate release in the absence of superfused cystine. Tonic glutamate release was not affected by blocking gap junctional hemichannels with 18α-glycyrrhetinic acid, blocking ATP receptors with pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS), blocking Ca2+-dependent exocytosis from neurones with Cd2+ or bafilomycin, blocking Ca2+-dependent release from glia with indomethacin, or blocking anion channels with 5-nitro-2-(3-phenylpropyl amino) benzoic acid (NPPB) or tamoxifen. However tonic glutamate release was reduced by 4,4′-diisothiocyanostilbene-2,2′-disulphonic acid (DIDS), and was potentiated by inhibiting astrocytic conversion of glutamate to glutamine with methionine sulfoximine. These data suggest that although cystine–glutamate exchange is present in the hippocampus it does not generate significant tonic release of glutamate when the extracellular [cystine] is at a physiological level, and that tonic glutamate release is at least partly from astrocytes and is mediated by a DIDS-sensitive mechanism. Theoretical calculations suggest that a significant fraction of tonic glutamate release in hippocampal slices could occur via diffusion of glutamate across lipid membranes.