GABAA-receptors in the brain display a striking structural heterogeneity, which is based on a multiplicity of diverse subunits. The allocation of GABAA-receptor subtypes to identified neurons is essential for an analysis of the functional significance of receptor heterogeneity. Among GABA-receptive neurons, well-characterized examples include the serotonergic and GABAergic neurons in the raphe nuclei. The GABAA-receptor subtypes expressed in these two types of neurons were analysed using antisera which recognize selectively the alpha 1- and alpha 3-subunits, and their co-localization with serotonin and glutamate decarboxylase was assessed by confocal laser microscopy in double and triple immunofluorescence staining in the rat. The vast majority of serotonergic neurons express strong alpha 3-subunit-immunoreactivity, but are devoid of alpha 1-subunit staining. In contrast, both the alpha 1- and alpha 3-subunit-immunoreactivities are present in glutamate decarboxylase-positive neurons. Thus, serotonergic and GABAergic neurons selectively express distinct patterns of alpha subunits, suggesting that they possess distinct subtypes of GABAA-receptors. The occurrence of neuron-specific GABAA-receptor subtypes may open new possibilities for the targeting of drugs with selective therapeutic actions.