The distribution of IL-6 mRNA in the rat brain was studied by in situ hybridization using 35S-labelled oligonucleotides. The mRNA encoding IL-6 was found in hippocampus, hypothalamus, cerebellum as well as in other brain areas. Microscopic analyses revealed both neuronal and glial cell localization of the mRNA. Subsequently, Northern blot analyses were performed with RNA isolated from spleen, adrenals, pituitary gland, hypothalamus, hippocampus and cerebellum of rats injected intraperitoneally with a non toxic dose of LPS. A rapid induction of the IL-6 mRNA was observed in the peripheral organs, whereas no change in IL-6 transcripts could be measured in the brain. It is concluded that the local synthesis and release of IL-6 in pituitary and adrenal gland might be involved in the activation of the HPA axis following an endotoxin challenge.