The involvement of GABA in neuronal differentiation and maturation precedes its role as inhibitory neurotransmitter in the brain. It was therefore investigated whether GABAA receptors mediating the actions of GABA in neonatal and adult brain can be distinguished by their molecular structure and cellular location. Immunohistochemistry with subunit-specific antibodies was employed to analyze changes in the distribution of GABAA-receptor subunits during postnatal development. In particular, subunit association patterns, as evidenced by colocalization of subunits within individual neurons, were analyzed by confocal laser microscopy. The subunits analyzed include the alpha 1- and alpha 2-subunits, which are associated with pharmacologically distinct GABAA-receptor subtypes, and the beta 2,3-subunits, which are a major constituent of GABAA receptors in both immature and adult rat brain. Each of these subunits exhibited age-dependent changes in their distribution, indicative of a differential maturation process. The alpha1-subunit immunoreactivity (-IR) was low at birth, restricted to a few areas, and increased dramatically during the first postnatal weeks. By contrast, the alpha 2-subunit-IR displayed a widespread distribution throughout the brain at birth, and disappeared from numerous areas soon after the appearance of the alpha 1-subunit. Double-immunofluorescence staining demonstrated the coexistence of both subunits in many individual neurons during a short time window, indicating that receptors containing the alpha 1-subunit gradually replace receptors containing the alpha 2-subunit in these cells. Staining for the beta 2,3-subunits was prominent and ubiquitous at every developmental age, indicating that these subunits are present in both neonatal and adult GABAA receptors. Indeed, double-immunofluorescence staining revealed an extensive colocalization of the alpha 2- and beta 2,3-subunits in neurons from neonatal rats, whereas the beta 2,3-subunits were associated with the alpha 1-subunit at later stages. Thus, the onset of alpha 1-subunit staining in maturing brain is indicative for the expression of a new, prevalent receptor subtype, presumably involved in synaptic inhibition. These findings demonstrate a switch in the subunit composition of GABAA receptors during postnatal development, suggesting the existence of molecularly distinct immature and adult forms of GABAA receptors in rat CNS.