ABSTRACTPrevious studies in rats have demonstrated that microsomal prostaglandin E synthase‐1 (mPGES‐1) is induced in brain vascular cells that also express inducible cyclooxygenase‐2, suggesting that such cells are the source of the increased PGE2 levels that are seen in the brain following peripheral immune stimulation, and that are associated with sickness responses such as fever, anorexia, and stress hormone release. However, while most of what is known about the functional role of mPGES‐1 for these centrally evoked symptoms is based on studies on genetically modified mice, the cellular localization of mPGES‐1 in the mouse brain has not been thoroughly determined. Here, using a newly developed antibody that specifically recognizes mouse mPGES‐1 and dual‐labeling for cell‐specific markers, we report that mPGES‐1 is constitutively expressed in the mouse brain, being present not only in brain endothelial cells, but also in several other cell types and structures, such as capillary‐associated pericytes, astroglial cells, leptomeninges, and the choroid plexus. Regional differences were seen with particularly prominent labeling in autonomic relay structures such as the area postrema, the subfornical organ, the paraventricular hypothalamic nucleus, the arcuate nucleus, and the preoptic area. Following immune stimulation, mPGES‐1 in brain endothelial cells, but not in other mPGES‐1‐positive cells, was coexpressed with cyclooxygenase‐2, whereas there was no coexpression between mPGES‐1 and cyclooxygenase‐1. These data imply a widespread synthesis of PGE2 or other mPGES‐1‐dependent products in the mouse brain that may be related to inflammation‐induced sickness symptom as well as other functions, such as blood flow regulation. J. Comp. Neurol. 522:3229–3244, 2014. © 2014 Wiley Periodicals, Inc.