Type A gamma-aminobutyric acid (GABA(A)) receptors are pentameric ligand-gated ion channels and the main drivers of fast inhibitory neurotransmission in the vertebrate nervous system1,2. Their dysfunction is implicated in a range of neurological disorders, including depression, epilepsy and schizophrenia3,4. Among the numerous assemblies that are theoretically possible, the most prevalent in the brain are the alpha 1 beta 2/3 gamma 2 GABAA receptors5. The beta 3 subunit has an important role in maintaining inhibitory tone, and the expression of this subunit alone is sufficient to rescue inhibitory synaptic transmission in beta 1-beta 3 triple knockout neurons6. So far, efforts to generate accurate structural models for heteromeric GABAA receptors have been hampered by the use of engineered receptors and the presence of detergents7-9. Notably, some recent cryo-electron microscopy reconstructions have reported ` collapsed' conformations8,9; however, these disagree with the structure of the prototypical pentameric ligand-gated ion channel the Torpedo nicotinic acetylcholine receptor10,11, the large body of structural work on homologous homopentameric receptor variants12 and the logic of an ion-channel architecture. Here we present a highresolution cryo-electron microscopy structure of the full-length human alpha 1 beta 3 gamma 2L-a major synaptic GABA(A) receptor isoformthat is functionally reconstituted in lipid nanodiscs. The receptor is bound to a positive allosteric modulator ` megabody' and is in a desensitized conformation. Each GABA(A) receptor pentamer contains two phosphatidylinositol-4,5-bisphosphate molecules, the head groups of which occupy positively charged pockets in the intracellular juxtamembrane regions of a1 subunits. Beyond this level, the intracellular M3-M4 loops are largely disordered, possibly because interacting post-synaptic proteins are not present. This structure illustrates the molecular principles of heteromeric GABA(A) receptor organization and provides a reference framework for future mechanistic investigations of GABAergic signalling and pharmacology.