During systemic inflammation, cytokines are released by immune‐competent cells into the circulation, which in turn signal the brain to mediate brain‐controlled signs of illness. Cytokine‐responsive brain cells can be mapped by histological analysis of cytokine‐induced transcription factors or transcription factor‐associated molecules revealing different cell phenotypes that respond to activation of the immune system. Critical sites mediating cytokine‐dependent immuneffector functions can be divided into two groups, one group of responding cells situated along a tight blood–brain barrier (BBB), and a second cell group in structures with an open BBB, e.g., the sensory circumventricular organs (CVOs). Previous reports from our group suggest that activation of the signal transducer and activator of transcription factor 3 (STAT3) during lipopolysaccharide (LPS)‐induced systemic inflammation is mediated by interleukin‐6 (IL‐6) and occurs in astrocytes of the rat CVOs. Here we show in the guinea pig a time‐dependent marked LPS‐induced STAT3 activation within astrocytes and endothelial cells of the CVOs, within astrocytes located in brain structures with a functional BBB and within the brain endothelium of the entire brain. In addition, systemic treatment of rats with either rat recombinant IL‐6 or LPS induced STAT3 activation in brain endothelial cells in a similar way as observed in the guinea pig brain, stressing the involvement of IL‐6 in this phenomenon in a more generalized way. The STAT3‐activated brain cells are located in critical target structures mediating cytokine action during LPS‐induced inflammation. STAT3‐controlled transcriptional activation with yet unknown cell‐specific functional consequences seems to be involved in this process. J. Comp. Neurol. 491:1–14, 2005. © 2005 Wiley‐Liss, Inc.