Light and electron microscopic immunohistochemistry was used to examine the time of emergence and distribution of GABA-containing cells in an association (prefrontal) and primary sensory (visual) region of the telencephalon at progressive fetal and postnatal stages of cortical maturation in the rhesus monkey. Thirty fetuses and six postnatal monkeys were examined beginning at embryonic day 41 (E41), the start of cortical neurogenesis, to birth (E165) and proceeding to maturity (greater than 5 years of age). The emergence and major developmental modifications in the distribution of immunoreactive neurons in both areas examined were nearly identical. GABA-immunolabeled neurons were present throughout the full thickness of the cerebral wall, including the cortical plate and the developmentally transient marginal, subplate, and ventricular zones, as early as E41. An important and surprising result was that a subset of bipolar migrating neurons in the subplate zone also contained GABA at these early stages. GABA-containing neurons in the ventricular and subventricular zones disappeared after E100, when neurogenesis is completed. In contrast, the number of immunoreactive multipolar and bipolar neurons within the subplate zone diminished between E100 and E131. By the first postnatal week, the distribution and density of GABA-containing neurons in the cortex appeared qualitatively similar to that observed in mature monkeys. The early appearance of GABA in cortical neurons and its expression by a population of migrating neurons suggest that a subset of cortical neurons may be committed to a transmitter phenotype independent of synaptic interactions and prior to attaining their adult positions in the maturing cerebral cortex.