Fresh biopsy specimens of human cerebral cortex were collected from patients suffering from deep-seated tumors requiring resection. GABAergic neurons were revealed in 50-microns-thick sections, for pre-embedding, and 1-micron-thick sections, for post-embedding GABA immunocytochemistry. In both thick and thin sections, the reaction product was found in neuronal cell bodies and in small profiles in the neuropil. In both preparations, GABA-containing somata were distributed evenly throughout the depth of the cortex. As best appreciated in the thicker sections, GABA-immunoreactive neurons belonged to a variety of morphological cell types with multipolar, bitufted or bipolar, and horizontal dendritic arbors. In the semi-thin sections sampled in the frontal cortex, the proportion of these neurons could be accurately evaluated and was found to be 21.2% +/- 4.8% of all cortical neurons. The average size of GABA-immunoreactive neurons was, in each layer, smaller than that of immunonegative neurons. The average soma size of both neuronal populations, immunoreactive and immunonegative for GABA, increased with depth. The comparison between the rat, cat, macaque monkey, and human GABAergic interneurons revealed similarities among primate brains, contrasting with the parameters (morphology, size, density) measured in rodents. These data are pertinent to the involvement of the GABAergic neurons in the shaping of receptive-field properties of cortical neurons in healthy brains and in pathologies involving the impairment of inhibitory neurotransmission.