The thalamic reticular nucleus (TRN) of the rat has been studied immunocytochemically using an antiserum against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Combined light and electron microscopic investigations by means of peroxidase-antiperoxidase and immunogold labeling show that this nucleus contains a homogeneous population of GABA-immunoreactive neurons receiving extensive GABAergic connections suggestive of self-inhibitory inputs.