PIP: This letter reports the structure of the major urinary metabolite formed from prostaglandin E2 (PGE2) in humans. Radiolabeled PGE2 was injected intravenously into male subjects. 50% of the radioactivity was recovered in urine during the first 5 hours and less than 3% during the following 12 hours. The urine was acidified, and this extract was subjected to reversed-phase partition chromatography. Formation of the major metabolite (depicted stereochemically in the text) from PGE2 involved 4 steps of reactions: 1) dehydrogenation of the alcohol group in the side chain; 2) reduction of the trans double bond; 3) 2 steps of beta oxidation; and 4) delta oxidation.