BGC 20-1531 is a potent and selective antagonist of the prostaglandin E2 receptor subtype 4 (EP4) with Ki values of 11.7, >10,000, and >10,000 nM for EP4, EP2, and EP3, respectively. It is selective for EP4, exhibiting<50% inhibition at 47 ion channels, cell-surface transporters, enzymes, and nuclear receptors at a concentration of 10 μM. BGC 20-1531 antagonizes PGE2-induced cAMP accumulation in a dose-dependent manner in HEK293 EBNA cells that stably express human EP4 receptors. It also reverses PGE2-induced vasorelaxation (Kb = 15.85 nM) in human middle cerebral and middle meningeal arterial rings. BGC 20-1531 prevents PGE2-induced increases in carotid blood flow and decreases in carotid resistance with an ID50 value of 5 mg/kg in dogs.