There are two schools of thought regarding the cyclooxygenase (COX) isoform active in the vasculature. Usingurinary prostacyclin markers some groups have proposed that vascular COX-2 drives prostacyclin release. Incontrast, we and others have found that COX-1, not COX-2, is responsible for vascular prostacyclin production. Ourexperiments have relied on immunoassays to detect the prostacyclin breakdown product, 6-keto-PGF1α andantibodies to detect COX-2 protein. Whilst these are standard approaches, used by many laboratories, antibodybasedtechniques are inherently indirect and have been criticized as limiting the conclusions that can be drawn. Toaddress this question, we measured production of prostanoids, including 6-keto-PGF1α, by isolated vessels and in thecirculation in vivo using liquid chromatography tandem mass spectrometry and found values essentially identical tothose obtained by immunoassay. In addition, we determined expression from the Cox2 gene using a knockin reportermouse in which luciferase activity reflects Cox2 gene expression. Using this we confirm the aorta to be essentiallydevoid of Cox2 driven expression. In contrast, thymus, renal medulla, and regions of the brain and gut expressedsubstantial levels of luciferase activity, which correlated well with COX-2-dependent prostanoid production. Thesedata are consistent with the conclusion that COX-1 drives vascular prostacyclin release and puts the sparseexpression of Cox2 in the vasculature in the context of the rest of the body. In doing so, we have identified thethymus, gut, brain and other tissues as target organs for consideration in developing a new understanding of howCOX-2 protects the cardiovascular system.