Interleukin-1 plays an important role as mediator of endotoxin-induced responses in the brain such as fever, sleep, anorexia, behavioural and neuroendocrine changes. In the present study, interleukin-1β immunocytochemistry has been performed at the light and electron microscopic level to study the cellular and subcellular localization of interleukin-1β in the brains of rats given endotoxin or saline. Light microscopic analysis of rats killed 4, 8 or 24 h after endotoxin (2.5 mg/kg) given intraperitoneally or intravenously revealed a region-specific localization of immunoreactive interleukin-1β in macrophages and microglial cells. After saline treatment, no induction of interleukin-1β immunoreactivity occurred in the brain. After administration of endotoxin, many interleukin-1β-positive cells were found in the meninges, choroid plexus, circumventricular organs, cerebral cortex and hypothalamus. The number of interleukin-1β-positive microglial cells reached a maximum 8 h after administration of endotoxin, irrespective of the route of administration. In general, more interleukin-1β-positive microglial cells were found after intravenous than after intraperitoneal administration of endotoxin. Interleukin-1β-positive microglial cells were often grouped in patches in the vicinity of blood vessels. At the surface of the cerebral cortex, in the meninges, intermediate cell forms between interleukin-1β-positive macrophages and microglial cells were found. interleukin-1β-positive perivascular microglia were localized at the brain side of the basal lamina. Immunoreactive interleukin-1β was found at the luminal side of the endothelial cells lining the venules. Furthermore, microglial cells that extended their processes into the ependymal layer of the third ventricle were observed. Results of the electron microscopic studies revealed immunoreactive interleukin-1β in many cells with the cellular characteristics of microglial cells, but also, in some cells, identified as astrocytes. In microglial cells, immunoreactive interleukin-1β was found in the cytoplasm but not in the endoplasmatic reticulum or Golgi apparatus. These results show that after peripheral administration of endotoxin, immunoreactive interleukin-1β is induced in macrophages in the meninges and in the choroid plexus, as well as in microglial cells in parenchyma. Interleukin-1β produced by these cells may serve as a signal for adjacent or more distant targets (neurons, endothelial cells, microglial cells) to play a role in the induction of non-specific symptoms of sickness.