During states of infection and inflammation, systemic interleukin 1 (IL-1) results in clear biological effects mediated through the central nervous system, such as fever, anorexia, and activation of the hypothalamic-pituitary-adreno-cortical axis. Given the multiple central effects of IL-1 in the rat, it would be expected that IL-1 receptors ought to be present in rat brain. However, no previous studies have localized IL-1 receptor or its mRNA in rat brain, possibly due to the fact that interspecies probes were used in previous studies. The recent cloning of the rat IL-1 type-I receptor (IL-1RI) has permitted us to conduct an in situ hybridization study using a species-specific, 35S-labeled anti-sense riboprobe to localize IL-1 receptor mRNA in rat brain. We localized IL-1RI mRNA in the hippocampus, choroid plexus, and cerebellum. At the cellular level we found IL-1RI mRNA in low to moderate levels in hippocampal neurons and in Purkinje cells of the cerebellar cortex, and in high levels in the endothelium of postcapillary venules and in glial cells surrounding arterioles throughout the brain. This pattern of localization provides support to the concept that IL-1 crosses the blood-brain barrier. Future studies are needed to clarify how the expression of the gene encoding IL-1RI is regulated in brain tissue.