Accumulating evidence supports the existence of an innate immune response in the brain during systemic inflammation that is associated with a robust induction of proinflammatory cytokines and chemokines by specific cells of the central nervous system. The present study investigated the genetic regulation and fine cellular distribution of the monocyte chemoattractant protein-1 (MCP-1) in the brain of mice and rats in response to systemic immune insults. MCP-1 belongs to a superfamily of chemokines that have a leading role in the early chemotaxic events during inflammation. In situ hybridization histochemistry failed to detect constitutive expression of the chemokine transcript in the cerebral tissue except for the area postrema (AP) that exhibited a low signal under basal conditions. This contrasts with the strong and transient induction of the mRNA encoding MCP-1 following a single systemic bolus of lipopolysaccharide (LPS), recombinant interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF-alpha). These stimuli rapidly triggered (30 to 90 minutes) MCP-1 transcription in all the circumventricular organs (CVOs), the choroid plexus (chp), the leptomeninges, and along the cerebral blood vessels. The time-related induction and intensity of the signal differed among the challenges, route of administration and species, but MCP-1-expressing cells were always found in vascular-associated structures and those devoid of blood-brain barrier. At later times, few isolated microglia across the brain parenchyma depicted positive signal for MCP-1 mRNA. A dual-labeling procedure also provided convincing anatomical evidence that endothelial cells of the microvasculature and a few myeloid cells of the CVOs and chp were positive for the transcript during endotoxemia. This gene is under a sophisticated transcriptional regulation, as the hybridization signal returned to undetectable levels 12 to 24 hours after all the treatments in both species. Of interest are the data that only ligands that triggered nuclear factor kappa B (NF-kappa B) signaling had the ability to increase MCP-1 gene expression, because high doses of IL-6 remained without effects. These data provide the anatomical evidence that MCP-1 is expressed within specific populations of cells in response to systemic inflammatory molecules that use NF-kappa B as intracellular signaling system. This chemokine may therefore play a critical role in the cerebral innate immune response and contribute to the early chemotaxic events during chronic cerebral inflammation.