Cyclooxygenase (COX) is an enzyme involved in the biosynthesis of prostaglandins and is one of the principle targets of non-steroidal anti-inflammatory drugs. Two isoforms of this enzyme are known to exist in the brain; one of these (type 1 COX or COX1) is constitutively expressed, whereas the other form of the enzyme, which is inducible, has been called type 2 COX (COX2). We have used systemic administration of bacterial lipopolysaccharide (LPS) as a model of the acute phase response to study the expression of COX2 in the murine CNS. We observed COX2 expression in neurons of several regions of the normal murine telencephalon. Robust expression of COX2 mRNA was induced in perivascular cells between 45 min and 6 h after LPS injection. The role of prostaglandins produced by these perivascular cells in the cerebral components of the acute phase response remains to be elucidated.