Interleukin-6 (IL-6) is a pleiotropic cytokine produced by various lymphoid and neural cells. In addition to its classic role during immune and inflammatory responses, IL-6 acts on the central nervous system to elicit changes, such as activation of the hypothalamic-pituitary-adrenal (HPA) axis. This study investigated the effects of systemic and central injection of IL-6 on neural activity and transcription of the corticotrophin-releasing factor (CRF) gene in the brain of conscious rats. The animals were killed 1 and 3 h after a single infusion of IL-6 into the right jugular vein (0.83 or 3.0 μg) or the right lateral ventricle (0.2 μg) and their brains cut from the olfactory bulb to the end of the medulla in 30-μm coronal sections. Messenger RNA encoding the protein Fos, a marker of neural activity, and the neuropeptide CRF were localized by in situ hybridization histochemistry using 35S-labelled exonic and intronic riboprobes. The results show that systemic injection of IL-6 induced specific transcription of c-fos gene in most of the sensorial circumventricular organs, including the organum vasculosum lamina terminalis, subfornical organ, median eminence, and area postrema, as well as in the central nucleus of the amygdala and bed nucleus of the stria terminalis. On the other hand, central injection of IL-6 increased cellular level of c-fos mRNA in the ependymal layer and the walls of the ventricles, meninges, nucleus of the solitary tract, and circumventricular organs. These effects were rapid and transient, since the signals for c-fos mRNA were detected 1 h after both treatments and vanished 3 h afterwards. Moreover, the CRF gene was not activated by either systemic or central administration of IL-6 in the paraventricular nucleus of the hypothalamus. Taken together, these results suggest that circumventricular organs hold a privileged position in mediating the central effects of systemic IL-6 and that centrally injected IL-6 can strongly activate cells of the ventricular system and surrounding structures. Although this differential circuitry may explain distinct origin-dependent functions of IL-6, this cytokine seems insufficient, in itself, to induce transcription of the gene encoding neuroendocrine CRF, the neuropeptide responsible for control of the HPA axis.