Cyclooxygenase 2 (COX-2) is a newly discovered isoform of cyclooxygenase that is inducible by lipopolysaccharide (LPS) or cytokines. This enzyme is considered to play a major role in inflammatory processes by catalyzing the production of prostaglandins. In the present study, induction of COX-2 mRNA in the rat brain by intraperitoneal injection of LPS was studied by the in situ hybridization technique with special attention paid to timing and sites of induction along with the time course of fever. In situ hybridization was carried out on sections of rat brain, 1 h (latent phase), 2.5 h (maximally febrile phase), 4 h (plateau phase), and 7 h (recovery phase) after the LPS injection, as well as on those from the brains of untreated and saline-injected rats. Injection of LPS induced COX-2 mRNA in the brain in two different constituents: neuronal cells and non-parenchymal cells of the blood vessels and leptomeninges. Induction in the neuronal cells was restricted to some telencephalic areas where the COX-2 mRNA signal was also detected in control animals. The signal was maximally enhanced by 50 to 80% over the basal level 1 h after LPS injection. The COX-2 mRNA signal was hardly detectable in neuronal and glial cells in other brain regions, including the preoptic area, either in control or LPS-injected rats. Strong COX-2 mRNA signals, however, appeared in the inner surface of blood vessels and the leptomeninges over the entire brain, including the preoptic area and its vicinity. The signals were not detectable in the brains of control rats and were most intense in the brains of rats treated with LPS for 2.5 h or 4 h. These results demonstrate that two major cell groups in the brain, neuronal cells and non-parenchymal cells, are responsible for the enhanced production of prostaglandins after systemic LPS treatment. Considering the site and timing of induction, we propose a possible role for blood vessels and leptomeninges as the source of prostaglandin E2 in the genesis of fever.