Prostaglandin D2 is synthesized by hematopoietic-type PGD-synthase (H-PGDS) in mast cells and is released in large quantities during allergic and asthmatic anaphylaxis. PGD2 is also produced in the brain by lipocalin-PGD-synthase also known as β-trace. In the brain, PGD2 produces normal physiological sleep and lowering of body temperature. Further pharmacological actions include inhibition of platelet aggregation and relaxation of vascular smooth muscle. tetranor-PGDM is a major metabolite of PGD2 that is detectable in human and mouse urine. The levels of tetranor-PGDM and 2,3-dinor-11β-PGF2α , a related PGD2 metabolite, in human urine were found to be 1.5 ± 0.3 and 0.6 ± ng/mg creatinine, respectively. tetranor-PGDM was detected in murine urine at a level of 8.1 ± 1.3 ng/mg creatinine.