Prostacyclin (PGI2), an important determinant of cardiovascular biology, is produced from arachidonic acid by endothelial cells. Measurement of its stable urinary metabolite, 2,3-dinor-6-oxo prostaglandin F1 alpha (PGI2-M), is the approach of choice to assess variations of the endogenous synthesis of PGI2 that occur in response to dietary, pharmacological and pathological alterations. We developed a relatively simple stable isotope dilution assay for PGI2-M which involves solid-phase extraction of 10 ml of urine with Chem Elut disposable columns, water/solvent partitioning from basic and acidic environments with ethyl acetate and methylene chloride, derivatization to 1-pentafluorobenzyl ester followed by TLC, methoximation and trimethylsilylation. Quantification was achieved, for the first time for PGI2-M, by capillary GC-electron capture negative ion MS-MS with a triple quadrupole mass spectrometer operated in the negative ion detection mode with methane as moderating gas. The mean inter-assay R.S.D., determined on 12 different urine samples, was 5.1 (range 0.4% to 10.5%). The excretion of PGI2-M in 34 healthy male subjects (age 26 to 57) was 156.2 +/- 65.2 (mean +/- S.D.) ng/24 h.