Our previous single unit and ultrastructural studies of visual cortex of dark-reared rats revealed an impairment of intracortical inhibitory mechanisms [2,3,5]. Neurochemical changes in inhibitory neurotransmitter and/or neuropeptides, such as gamma-aminobutyric acid (GABA) and somatostatin (SS), respectively, may contribute to the observed alterations. The present study was designed to measure GABA and SS alterations in the visual cortex of the same dark-reared preparation, as possible neurochemical correlates of the changes seen both physiologically and anatomically in previous companion studies. In the present investigation the mean densities of GABA- and SS-immunoreactive neurons in area 17 of dark-reared rats were determined and compared to the density of those of rats reared in normal lighting conditions. Dark-rearing resulted in a significant decrease in the density of GABA-immunoreactive neurons in all cell layers of area 17 of the rat visual cortex; not limited to the thalamorecipient layer(s). There was also a higher mean density of total cortical cells in dark-reared animals. No differences, however, were seen in the density of SS-immunoreactive neurons. The alterations of GABA-immunoreactive neurons in all cortical layers agree with the altered synaptic ultrastructure and physiological responses seen in all cortical layers as reported in our previous companion studies. Taken together, these studies further support the notion of a deficit in intracortical inhibitory mechanisms in the visual cortex of dark-reared adult rats.