The distribution of interleukin 6 (IL-6) mRNA and IL-6 receptor (IL-6R) mRNA in the brain of adult male rats was studied at the light microscope level by in situ hybridization histochemistry using 35S-labelled oligonucleotides. The transcripts of both genes were localized in the pyramidal neurons and in the granular neurons of the hippocampus, in neurons of the habenular nucleus as well as in the dorsomedial and ventromedial hypothalamus, in the piriform cortex, in scattered neurons of the cortex and in granular cells of the cerebellum. The medial preoptic nucleus and the anterior tip of the lateral ventricle contained mRNA encoding IL-6 and its receptor. Moreover, white matter areas, such as the internal capsule, which consist of only fibres and glial cells, were found to have autoradiographic signals above background. The mRNAs for IL-6 and IL-6R in hippocampus and cerebellum are not different, as shown by Northern blot analyses of RNA isolated from these tissues. We postulate that the cytokine IL-6 is expressed constitutively in discrete regions of the CNS and that it is involved in the mechanisms coordinating metabolic, behavioural and neuroendocrine changes not only during illness but also under normal physiological conditions. Our results suggest that IL-6 mRNA and IL-6R mRNA are colocalized, thus supporting a role of the cytokine in autocrine and paracrine communication.