Systemic interleukin‐1 (IL‐1) activates the hypothalamo‐pituitary‐adrenal (HPA) axis, an effect exerted through increased synthesis and secretion of corticotropin‐releasing factor (CRF) by parvicellular neurosecretory neurons. The site(s) and mechanism(s) through which circulat ing IL‐1 may access central systems governing HPA axis output remain obscure. To identify potential cellular targets for blood‐borne IL‐1, we analyzed the distribution of mRNA encoding the rat type 1 IL‐1 receptor (IL‐1R1) in rat brain. Regional ribonuclease protection assays detected a single protected fragment corresponding to the membrane‐bound form of the IL‐1R1 mRNA in all areas analyzed. In situ hybridization revealed labeling predominantly over barrier‐related cells, including the leptomeninges, non‐tanycytic portions of the ependyma, the choroid plexus, and vascular endothelium. Low to moderate levels of the IL‐1111 mRNA were detected in just a few neuronal cell groups, including the basolateral nucleus of the amygdala, the arcuate nucleus of the hypothalamus, the trigeminal and hypoglossal motor nuclei, and the area postrema. No specific labeling for IL‐1R1 mRNA was detected over neurons that respond to intravenous IL‐1β; by induction of transcription factor Fos, including hypophysiotropic CRF cells and brainstem catecholamine neurons. Injection of IL‐1β did, however, provoke induction of mRNA encoding the immediate‐early gene, NGFI‐B, but not c‐fos, in two major loci of IL‐1R1 expression, vascular endothelial cells, and the area postrema. Intravenous inection of IL‐1β acutely down‐regulated IL‐1R1 mRNA in perivascular cells, but not in neuronal cell groups. These results suggest the parenchymal sites of IL‐1R1 expression in rat to be distinct from those reported previously in mouse. The common expression in both species of an IL‐1R in non‐neuronal elements highlights the possibility that IL‐1‐mediated activation of CRF neurons may result from cytokine‐receptor interaction at vascular, and/or other barrier‐related, sites to trigger release of secondary signalling molecules in a position to interact with components of HPA control circuitry. © 1995 Wiley‐Liss, Inc.