The gamma-aminobutyric acid (GABA)ergic properties of embryonic (E15d) rat cortical neurons were studied in dissociated serum-free culture by immunohistochemical methods. GABA-like immunoreactivity was found in a subpopulation of neurons from the first day onwards. The number of GABA-positive neurons reached mature values (10.5-12.6%) within the first week, while their morphological differentiation was not found to be fully completed until the 11th day of culture and was characterized by several discrete developmental stages. First, GABA-positive neurons gained their mature complement of neurites at 3 days in vitro (DIV). Three days later somal maturation became evident, followed at least by the maturation of the neuritic arbor. Double-labelling studies revealed the coexpression of GABA and tyrosine hydroxylase within the same cells. The similarities of relative number, morphology, time course of development and biochemistry of cultured GABAergic neurons compared with those in situ suggest that the applied culture system is a useful model to investigate several aspects of GABAergic neurotransmission at the cellular level.