The cellular source of interleukin-6 (IL-6) in brain has not been identified. Using a sensitive, non-radioactive in situ hybridization technique, the cellular localization of IL-6 mRNA in different areas of the rat brain was analysed. Intense staining for IL-6 mRNA was found in Purkinje cells and hippocampal neurons. Our data support an essential role of IL-6 for cerebellar and hippocampal neuron differentiation and function.