The inflammatory cytokine interleukin-1 has been implicated as a mediator of many centrally controlled responses, such as fever and increased activity of the hypothalamic-pituitary-adrenal axis, after systemic infections. To identify the neuroanatomical loci of brain interleukin-1-producing cells during infection, we investigated interleukin-1 beta messenger RNA expression by ill situ hybridization histochemistry using a 500 nt ribonucleotide probe applied on brain sections from rats injected intraperitoneally with 2.5 mg/kg bacterial lipopolysaccharide or saline. In control animals, interleukin-1 beta messenger RNA was not detectable. In the brains of lipopolysaccharide-injected animals, two temporally and spatially distinct waves of interleukin-1 beta messenger RNA induction were observed. First, cell labelling appeared at 0.5 h, peaked at 2 h, and declined at 4-8 h. The labelled cells were concentrated in circumventricular organs organum vasculosum of the lamina terminalis, subfornical organ, median eminence, and area postrema and in choroid plexus, meninges, and blood vessels. Second, at 8-12 h, scattered small cells became labelled throughout the entire brain parenchyma; the labelling subsided by 24 h. Labelling was not observed in any neurons. In the pituitary, lipopolysaccharide induced strong interleukin-1 beta messenger RNA expression initially in the anterior lobe at 0.5-1 h, and later in the neural lobe at 1-2 h, and subsiding thereafter. The results show that at early rime points, peripheral lipopolysaccharide induces interleukin-1 beta message production at the blood-brain barrier and in circumventricular organs where the blood-brain barrier is leaky. After a time delay of 6-10 h, however, interleukin-1 beta messenger RNA is primarily expressed by non-neuronal cells of the brain in the brain parenchyma. These results suggest that the source of initial brain IL-1 activity after peripheral lipopolysaccharide injection derives from cells of the blood-brain barrier and the circumventricular organs, and the sustained interleukin-1 activity in the central nervous system thereafter is derived from glia . Published by Elsevier Science Ltd.