The γ2 subunit is necessary for the expression of the full benzodiazepine pharmacology of GABAA receptors and is one of the major subunits in the brain. In order to determine the location of channels containing the γ2 subunit in relation to GABA-releasing terminals on the surface of neurons, a new polyclonal antipeptide antiserum was developed to the γ2 subunit and used in high resolution, postembedding, immunoelectron-microscopic procedures. Dual immunogold labelling of the same section for two subunits, and up to three sections of the same synapse reacted for different subunits, were used to characterize the subunit composition of synaptic receptors. The γ2 subunit was present in type 2, “symmetrical” synapses in each of the brain areas studied, with the exception of the granule cell layer of the cerebellum. The γ2 subunit was frequently co-localized in the same synaptic junction with the α1 and β23 subunits. The immunolabelling of synapses was coincident with the junctional membrane specialization of the active zone. Immunolabelling for the receptor often occurred in multiple clusters in the synapses. In the hippocampus, the γ2 subunit was present in basket cell synapses on the somata and proximal dendrites and in axo-axonic cell synapses on the axon initial segment of pyramidal and granule cells. Some synapses on the dendrites of GABAergic interneurones were densely labelled for the γ2, α1 and β23 subunits. In the cerebellum, the γ2 subunit was present in both distal and proximal Purkinje cell dendritic synapses established by stellate and basket cells, respectively. On the soma of Purkinje cells, basket cell synapses were only weakly labelled. Synapses on interneuron dendrites were more densely labelled for the γ2, α1 and β23 subunits than synapses on Purkinje or granule cells. Although immunoperoxidase and immunofluorescence methods show an abundance of the γ2 subunit in granule cells, the labelling of Golgi synapses was much weaker with the immunogold method than that of the other cell types. In the globus pallidus, many type 2 synapses were labelled for the γ2 subunit together with α1 and β23 subunits. The results show that γ2 subunit-containing receptor channels are highly concentrated in GABAergic synapses that also contain the α1 and β23 subunits. Channels containing the γ2 subunit are expressed in synapses on functionally distinct domains of the same neuron receiving GABA from different presynaptic sources. There are quantitative differences in the density of GABAA receptors at synapses on different cell types in the same brain area. Copyright © 1996 Elsevier Science Ltd