Monoclonal antibodies to a synthetic peptide fragment of the beta 1-subunit of the bovine central GABAA/benzodiazepine receptor were used to investigate immunocytochemically the distribution of this receptor in the visual system of the cat. Labeled neurons were observed in all layers of the visual cortex and the dorsal lateral geniculate nucleus. About half of the total cortical or geniculate neuronal population was found to be positive. To further identify immunocytochemically these GABAA receptor expressing cells, double stainings were undertaken with, on one hand, the monoclonal antibodies directed against the receptor complex, and on the other hand polyclonal antisera directed against cat muscle parvalbumin or chicken calbindin D-28K. A high degree of colocalization between either of the two calcium binding proteins and the GABAA receptor was found in the upper layers (I, II and III) of the visual cortex and in the A and C laminae of the dorsal lateral geniculate nucleus; all calbindin D-28K-positive cells were immunoreactive for the GABAA receptor. The parvalbumin-positive cells, scattered throughout all layers of the dorsal lateral geniculate nucleus and the visual cortex, except cortical layer I, were also all positive for the GABAA receptor. However, a large proportion of all GABAA receptor bearing cells were negative for one of the calcium binding proteins.