Prostaglandin E₂ (PGE₂), the brain mediator of fever, is thought to be produced in brain endothelial cells, as these cells express PGE₂-synthesizing enzymes such as cyclooxygenase-2 (COX-2) and microsomal-type PGE synthase in response to pyrogen lipopolysaccharide (LPS). However, it is not directly demonstrated yet if these cells actually produce PGE₂ in response to LPS, and which type of phospholipase A₂ (PLA₂) is working in the upstream of COX-2. To solve these issues, we here established an ex vivo model of PGE₂ production in brain blood vessels. Subarachnoidal blood vessels together with arachnoidal membrane were isolated from the rat brain 4 hrs after intraperitoneal injection of either saline or LPS (400 mg/kg). The samples were incubated in HEPES-buffered Ringer solution for one hour at 37°C in the presence or the absence of various enzyme inhibitors, and concentrations of PGE₂ in the incubation medium were measured using EIA. PGE₂ production was significantly enhanced in blood vessels from LPS-treated rats compared to those from saline-treated ones. A COX-2 inhibitor added to the incubation medium significantly suppressed the PGE₂ production. Since endothelial cells are the major COX-2 expressing cells there, these results directly indicate that brain endothelial cells enhance PGE₂ production in response to peripheral LPS stimulus. This ex vivo model could be further useful to examine the type of PLA₂ involved in PGE₂ production during fever. [Jpn J Physiol 54 Suppl:S231 (2004)]