Prostaglandin D2 (PGD2) plays important roles in vascular smooth muscle relaxation, the inhibition of platelet aggregation, and anaphylaxis. 13,14-dihydro-16,16-difluoro PGD2 is an analog of PGD2. While its biological activities have not been evaluated, it should be noted that the addition of two electron-withdrawing fluorine atoms has been used to stabilize prostanoids and significantly delay degradation in vivo.{17623} Importantly, 13,14-dihydro PGE1 has activity that is comparable to that of PGE1, suggesting that this analog of PGD2 could be biologically active.{4168,182}