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Glycine receptors (GlyRs) are pentameric, 'Cys-loop' receptors that form chloride-permeable channels and mediate fast inhibitory signalling throughout the central nervous system1,2. In the spinal cord and brainstem, GlyRs regulate locomotion and cause movement disorders when mutated2,3. However, the stoichiometry of native GlyRs and the mechanism by which they are assembled remain unclear, despite extensive investigation4-8. Here we report cryo-electron microscopy structures of native GlyRs from pig spinal cord and brainstem, revealing structural insights into heteromeric receptors and their predominant subunit stoichiometry of 4α:1β. Within the heteromeric pentamer, the β(+)-α(-) interface adopts a structure that is distinct from the α(+)-α(-) and α(+)-β(-) interfaces. Furthermore, the β-subunit contains a unique phenylalanine residue that resides within the pore and disrupts the canonical picrotoxin site. These results explain why inclusion of the β-subunit breaks receptor symmetry and alters ion channel pharmacology. We also find incomplete receptor complexes and, by elucidating their structures, reveal the architectures of partially assembled α-trimers and α-tetramers.