11 alpha-Hydroxy-9,15-dioxo-2,3,4,5,20-pentanor-19-carboxyprostano ic acid (PGE-M) and 9 alpha,11 alpha-dihydroxy-15-oxo-2,3,4,5,20-pentanor-19-carboxyprostanoic acid (PGF-M) in urine were determined in an isotope dilution assay by gas chromatography/triple-stage quadrupole mass spectrometry. After addition of the 2H7-labeled internal standard, O-methylhydroxylamine hydrochloride in acetate buffer was added either directly (PGE-M) or after standing overnight at pH 10 (PGF-M) to form the methoxime. The sample was acidified to pH 2.5 and PGE-M and PGF-M were extracted with ethyl acetate/hexane. Then the prostanoids were derivatized to the pentafluorobenzyl ester and purified by thin-layer chromatography and the trimethylsilyl ether was formed. The products were quantified by gas chromatography/triple-stage quadrupole mass spectrometry. For PGE-M, the fragment ions m/z 349 and m/z 356 (2H7 standard) (daughter ions of m/z 637 and m/z 644 (2H7 standard] were used. The results of the PGE-M assay were compared with those of an assay using the [2H3]methoxime as the internal standard. For determination of PGF-M, the daughter ions m/z 484 and m/z 491 (2H7 standard) with the parent ions m/z 682 and m/z 689 (2H7 standard) were chosen.