The aim of the present study was to assess potential brain sites of stimulation by peripheral interleukin (IL)-6 of the hypothalamo-pituitary-adrenal (HPA) axis in the rat, using c-fos protein as a marker of cellular activation. Involvement of prostaglandins in IL-6-induced ACTH secretion and c-fos expression was also investigated. IL-6 was infused continuously (40 ng/min) for 90 min to conscious male rats. Blood samples were taken before the infusion and at 30 and 90 min for measurement of plasma ACTH. Expression of c-fos in the brain was examined by immunohistochemistry. Administration of IL-6 significantly elevated plasma ACTH levels at 30 min (495 ± 105 vs. 117 ± 17 pg/ml in controls, p < 0.05). Elevated levels were still present at 90 min (596 ± 139 vs. 113 ± 20 pg/ml in controls, p < 0.05). Infusion of IL-6 (3.6 µg/rat) markedly triggered c-fos expression in hypothalamic paraventricular (PVN) and supraoptic nuclei (SON), as well as in the central amygdaloid nucleus (CeA), the nucleus tractus solitarius and the locus coeruleus. Pretreatment with the cyclooxygenase inhibitor indomethacin (10 mg/kg, i.v.) suppressed the ACTH response induced by IL-6. The number of IL-6-induced immunoreactive cells in the PVN was significantly reduced by indomethacin pretreatment (p < 0.01), but the number of IL-6-induced c-fos-positive cells in the SON and CeA remained unchanged. These findings suggest that circulating IL-6 may exert central actions by acting directly or indirectly on brain neurons. In addition, the ability of IL-6 to activate the HPA axis may depend upon the release of prostaglandins, probably in the brain.