Serial sections of the cat's thalamus were incubated with a purified antiserum raised against gamma-aminobutyric acid conjugated to bovine serum albumin by distilled glutaraldehyde. This serum has been extensively characterized and appears to react selectively with fixed gamma-aminobutyric acid in brain tissue treated with glutaraldehyde. Adjoining sections were stained with thionin and served as invaluable guides for a correct evaluation of the immunolabelling pattern. In the neuropil the intensity of the immunostaining varies considerably between thalamic nuclei and even between nuclear subdivisions. The neuropil staining appears particularly dense in the nuclei parataenialis, periventricularis, centralis medialis, reuniens, rhomboideus, habenularis lateralis, centrum medianum, parafascicularis, subparafascicularis, submedius, dorsal and ventral parts of the lateral geniculate body, the dorsal part of the medial geniculate body, the posterior complex, suprageniculate nucleus, pulvinar and parts of the lateral posterior nucleus. The pulvinar/lateralis posterior complex shows a particularly well-differentiated staining pattern which closely matches Updyke's [Updyke (1983) J. comp. Neurol. 219, 143-181] parcellation of this region. In several thalamic nuclei or subareas--and notably in those relay nuclei which are known to project upon non-primary sensory cortical areas--the immunostained neuropil is characterized by many puncta encircling an unstained profile. With few exceptions all thalamic nuclei displayed immunoreactive nerve cell bodies. Several examples were found of a mismatch between the number of such cells and the staining intensity of the neuropil. Thus the nuclei periventricularis, parafascicularis, subparafascicularis, parataenialis, limitans and centrum medianum although being very rich in neuropil staining have practically no immunostained perikarya. Rough estimates were made of the size and the proportion of gamma-aminobutyric acid labelled neurons in all major--and some minor--thalamic nuclei and their subdivisions. In some thalamic nuclei, notably the nuclei reticularis, anterior dorsalis, lateralis dorsalis, centralis lateralis, ventralis posterior and the dorsal lateral geniculate body, the population of immunoreactive neurons is distinctly heterogeneous with regard to soma size. The findings are discussed with regard to previous immunocytochemical studies of the distribution of gamma-aminobutyric acid and its synthesizing enzyme in the thalamus. Particular emphasis is put on the great species differences which appear to exist in this respect.