Using the sensitive technique of reverse transcription followed by polymerase chain reaction (RT-PCR), the expression of interleukin-6 (IL-6) and its receptor (IL-6R) in rat hypothalamus was investigated. Our results show a differential expression of the mRNAs for IL-6 and IL-6R during postnatal development: whereas IL-6 mRNA slightly declines with increasing age, IL-6R mRNA levels markedly increase (4-fold). The time course of these changes correlates with the maturation of the hypothalamo-pituitary-adrenocortical axis, indicating an involvement of the IL-6 system in neuroendocrine regulation. Expression of both genes in other brain areas suggest additional functions in the central nervous system (CNS).