The prostaglandin 11-keto reductase of rabbits, which catalyzes the conversion of prostaglandin D2 to prostaglandin F2 alpha, is only found in the liver. This enzyme, which is primarily localized in the soluble fraction, requires NADPH for activity; its Km value for prostaglandin D2 is approx. 200 muM. [14C]Prostaglandin D2 administered to rabbits via the portal vein escapes from the liver unchanged, suggesting that exogenous prostaglandin D2 may be inaccessible to the enzyme. The possible role of this enzyme in the metabolism of endogenous hepatic prostaglandin D2 is suggested by the demonstration that the liver synthesizes prostaglandin D2 from arachidonic acid.