Prostaglandins are 2O-carbon-atom fatty acids that have been implicated in numerous pharmacological, physiological and pathological processesi. Most of their diverse e&&s can be evoked by an extremely small amount and many assay methods have been developed for their quantitative and qualitative analysis, e.g., thin-layer chromatography', enzyme assay3, fluorescence4, radioimmunoassay5, mass spectrometry (MS) and bioassay using a smooth muscle preparation6 or human platelet aggregation'. Of these methods, MS is the most accurate for the determination of prostaglandins and a number of studies have been concerned with their determination by this technique, especially electron-impact (EI) MS combined with gas chromatography (GC)*='. The biological significance of thromboxanes and prostaghmdin I, has recently become apparent. Thromboxane I& was first isolated and identified by Hamberg and Samuelssonlo after incubation of [l'C]arachidonic acid with guinea-pig lung homogenates. The substance was later revealed to be an end metabolite of the labile compound thromboxane A,, which has striking properties for inducing the aggregation of human platelets and vasocontraction ll. Prostaglandin I2 was recently shown by Moncada et ~1.l~ to be a labile substance generated by the arterial walls, with a striking capacity for the inhibition of platelet aggregation and vasodilation. Prostaglandin I2 was shown to be rapidly decomposed non-enzymatically into Gketoprostaglandin F, in biological &idP. 'IIre MS detection of thromboxane B2 and 6-ketoprostaglandin F1= has been reported by Hamberg and SamuelssonLo and Fenwick et aZ.14, respectively. However, both groups failed to show a molecular ion as the base peak, as they used El-MS, which generally $ves many fragment ions with extremely low intensity in the high mass range. This paper describes the use of ammonia chemical-ionization (CI) MS to identify the molecular weights of thromboxane B, and dketoprostaglandin Fla.