Prostaglandin (PG)D(2), an important mediator in allergic diseases, is rapidly transformed in plasma to active metabolites that bind and activate two distinct receptors, DP1 and CRTH2. Since the rate of PGD(2) degradation and the bioactivity of the resulting metabolites are still unclear, the aim of our study was to analyze the kinetics and biological effects of PGD(2) metabolites formed in plasma. Eosinophil shape change was taken as a parameter of chemotactic activation mediated by CRTH2 whereas inhibition of platelet aggregation served as a measure of DP1 activity. PGD(2) was degraded in plasma with an apparent half-life of approximately 30 min, accompanied by a loss of potency in inhibiting platelet aggregation as well as inducing eosinophil stimulation. Incubation of PGD(2) in plasma for 120 min caused an increase in the IC(50) for platelet aggregation by a factor of 6.5 and an increase of the EC(50) for eosinophil shape change by a factor of 7.2. However, tandem mass spectrometry analysis showed that incubation of PGD(2) in plasma for 120 min resulted in clearance of PGD(2) of more than 92%, which was mirrored by a continuous formation of Delta(12)-PGD(2) and Delta(12)-PGJ(2), whereas only small amounts of 15d-PGD(2) and 15d-PGJ(2) were detected. Interestingly, a rapid degradation of PGD(2) was also observed in serum, which was not prevented by pepsin digestion of serum preceding the addition of PGD(2). Therefore, despite extensive non-enzymatic metabolization of PGD(2) in plasma, its biological activity with respect to DP1 and CRTH2 is maintained through the formation of bioactive metabolites.