The areal and laminar distribution of GABAA receptor immunoreactivity was examined in fetal, early postnatal and adult monkey sensory-motor cortex by using a monoclonal antibody to the purified GABAA receptor complex (Vitorica et al. 1988). GABAA receptor immunoreactivity was distributed throughout the neuropil, often outlining the unstained somata of pyramidal and non-pyramidal cells. In all areas of the adult sensory-motor cortex, layers I-IIIA exhibited the most intense immunostaining. In deeper layers of the four cytoarchitectonic fields of the first somatic sensory area (SI), layers IIIB and V were lightly stained and alternated with somewhat more intensely stained layers IV and VI. In deeper layers of area 4, the deeper half of layer IIIA through layer VA was lightly immunostained, but layers VB and VI were slightly more intensely immunoreactive. A variable number of nonpyramidal cell somata in the cortex and underlying white matter showed immunoreactive staining. GABAA receptor immunoreactivity was present throughout the sensory-motor cortex from the youngest fetal age examined (E121), but the pattern of immunostaining differed from that in the adult. In all areas, the densest immunoreactivity was found in a diffuse band in layers III and IV and in the subplate zone. Within the subplate zone, the presence of receptor immunoreactivity and some intensely stained neuronal somata at all fetal ages suggests the presence of a synaptic neuropil. With increasing age, gradual changes in the distribution of receptor immunoreactivity occurred, resulting in an adult-like pattern of immunostaining by postnatal day 1.5. These results show that laminar pattern of GABAA receptor distribution closely follows the major concentrations of GABA immunoreactive neurons in adults and it is suggested that laminar changes seen in development are associated with the establishment of afferent connections and inhibitory circuits in the sensory-motor cortex.