Recent investigations have reported a reduced density of interneurons and an increase of GABAA receptor binding occurring preferentially in layer II of the anterior cingulate cortex of schizophrenic subjects [Benes F.M. et al. (1992) J. Neurosci. 12, 924-929]. Since a reduction in the density of interneurons has also been found in layer II of the prefrontal cortex, this study has sought to determine whether an un-regulation of the GABAA receptor binding activity might also be found in this region of schizophrenics. A high-resolution autoradiographic analysis of bicuculline-sensitive [3H]muscimol (GABAA) receptor binding on individual neuron cell bodies in layers II, III, IV and VI has been applied to Brodmann area 10 from normal controls (n = 16) and schizophrenic (n = 7) subjects. A computer-assisted technique has been used under strictly blind conditions to determine whether differences in binding occur in the schizophrenic group. A significant increase of GABAA receptor binding activity has been observed in layers II, III, V and VI in the schizophrenic group. When the binding is expressed as a density with respect to neuronal cell size, there is a gradient of binding across layers II, III, V and VI, with neuronal cell bodies in layer II having the greatest density of grains. When different subpopulations of neurons distinguished according to size criteria are examined separately, large (pyramidal) neurons show significantly higher binding, particularly in layer II, where it was increased by 90% in schizophrenics. Small (non-pyramidal) cells do not show significant differences in binding in schizophrenics, except in layer VI, where there was a 135% increase. Potential confounding effects from age and post mortem interval do not explain the differences between the two groups, because both young and old schizophrenics, as well as schizophrenics with long and short post mortem intervals, showed increased GABAA receptor binding activity when compared to control cases distinguished in a corresponding fashion. These data suggest that there may be a preferential reduction of inhibitory GABAergic inputs to pyramidal neurons, particularly in layer II of the preferential cortex, in schizophrenia. This change could potentially result in an increased excitatory outflow from the prefrontal area to other cortical regions of the schizophrenic brain. Overall, these results are consistent with the idea that reduced amounts of GABAergic activity in the prefrontal cortex could be related to a perinatal disturbance and could be a potentially important component of the pathophysiology of psychosis.