A study of neurons and processes (puncta) immunolabeled by antibodies to gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase was undertaken in the medial geniculate body of the adult cat. The proportion and types of GABAergic cells were determined with high resolution methods, including postembbedding immunocytochemistry on semithin plastic sections. A second goal was to draw parallels and differences between the auditory thalamus and other thalamic nuclei. Finally, the types of GABAergic puncta and their concentration in the three major subdivisions of the medial geniculate body were analyzed. The results were that (1) each division had many GABAergic neurons, averaging approximately 26% of the neuronal population; (2) the ventral division had the highest proportion of these cells (33%), the medial division the fewest (18%), and the dorsal division was intermediate (26%); (3) there was a gradient in the proportion of GABAergic neurons, i.e., the ventral and medial division values increased caudorostrally, whereas the value in the dorsal division declined; (4) the predominant GABAergic cell type in each division was a small neuron with a soma approximately 10-12 microm in diameter; (5) a small population of much larger GABAergic neurons was present mainly in the dorsal division; (6) in addition to the fine, granular puncta in each division, a type of giant GABAergic puncta was found only in the dorsal division nuclei. The results obtained with the two antibodies were essentially identical. These findings suggest a structural basis for qualitative differences in the distribution of GABAergic processing within the medial geniculate complex. The GABAergic arrangement in the ventral division was stereotyped, with only one type of putative GABAergic interneuron, and the puncta were correspondingly homogeneous. In contrast, the dorsal division had two types of GABAergic neurons, and the giant GABAergic puncta represent a new substrate for inhibitory interactions. The medial division also had more than one type of GABAergic neuron and a slightly lower concentration of puncta. These qualitative and quantitative distinctions suggest a morphologic basis for possible differences in inhibitory processing among medial geniculate body subdivisions.