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Dysregulation of γ‐aminobutyric acidergic (GABAergic) transmission has been reported in lesional acquired epilepsies (gliomas, hippocampal sclerosis). We investigated its involvement in a developmental disorder, human focal cortical dysplasia (FCD), focusing on chloride regulation driving GABAergic signals. Methods In vitro recordings of 47 human cortical acute slices from 11 pediatric patients who received operations for FCD were performed on multielectrode arrays. GABAergic receptors and chloride regulators were pharmacologically modulated. Immunostaining for chloride cotransporter KCC2 and interneurons were performed on recorded slices to correlate electrophysiology and expression patterns. Results FCD slices retain intrinsic epileptogenicity. Thirty‐six of 47 slices displayed spontaneous interictal discharges, along with a pattern specific to the histological subtypes. Ictal discharges were induced in proepileptic conditions in 6 of 8 slices in the areas generating spontaneous interictal discharges, with a transition to seizure involving the emergence of preictal discharges. Interictal discharges were sustained by GABAergic signaling, as a GABAA receptor blocker stopped them in 2 of 3 slices. Blockade of NKCC1 Cl− cotransporters further controlled interictal discharges in 9 of 12 cases, revealing a Cl− dysregulation affecting actions of GABA. Immunohistochemistry highlighted decreased expression and changes in KCC2 subcellular localization and a decrease in the number of GAD67‐positive interneurons in regions generating interictal discharges. Interpretation Altered chloride cotransporter expression and changes in interneuron density in FCD may lead to paradoxical depolarization of pyramidal cells. Spontaneous interictal discharges are consequently mediated by GABAergic signals, and targeting chloride regulation in neurons may be considered for the development of new antiepileptic drugs. Sin financiación 9.037 JCR (2019) Q1, 9/204 Clinical Neurology, 18/271 Neurosciences 4.900 SJR (2019) Q1, 6/378 Neurology (clinical), 1/164 Neurology No data IDR 2019 UEM