Neurons accumulating [3H]gamma-aminobutyric acid (GABA) were identified autoradiographically in frozen and plastic sections after injection of the material in the monkey somatic sensory (areas 3, 1, and 2), motor (area 4), and parietal (area 5) cortex following intravenous administration of a GABA transaminase inhibitor, amino-oxyacetic acid. Two general forms of labeled cells are recognized: those with large diameter (15- to 20-micrometer) somata, probably corresponding to the basket cells of Golgi studies, and those with small diameter (6- to 12- micrometer) somata, probably corresponding to several other types. The sizes and laminar distributions of labeled cells in the cytoarchitectonic fields of the first somatic sensory are (SI) are similar. In these areas, the greatest concentrations of labeled cells are in layer II, in layer IV, and in the superficial stratum of layer IV. In area 5, there are many fewer labeled cells in layer IV. In area 4, the labeled cells are larger and distributed more homogeneously, though there are fewer in layer VI. The large labeled cell type is concentrated in layers III and V of all areas. The smaller labeled types are found in all other layers, including layer I. In layer IV, they form approximately 40% of the total small-celled neuronal population.