13,14-dihydro Prostaglandin E2 -13,14-dh-15-keto PGE2) is the primary metabolite of PGE2 in plasma. Endogenous or infused PGE2 is rapidly metabolized by the enzymes 15-hydroxy PGDH and 15-oxo-prostaglandin .DELTA.13-reductase to form 13,14-dh-15-keto PGE2. 13,14-dh-15-keto PGE2 accumulates to detectable levels; plasma levels in humans are between 10-100 pg/ml. It undergoes further metabolism and chemical decomposition, giving it a relatively short half-life. In dogs, the plasma half-life of 13,14-dh-15-keto PGE2 is about 9 minutes. In humans the metabolite has a similar short half-life, making it a poor choice of analytes for assays designed to measure total PGE2 biosynthesis.