Measurement of 7alpha-hydroxy-5,11-diketotetranoprostane-1,16-dioic acid, (PGE-M), the major urinary metabolite of prostaglandin E1 and E2 in man provides a useful indicator to monitor prostaglandin biosynthesis. For quantitative analysis of this prostaglandin metabolite and the stable-isotope dilution techniqe of selected ion monitoring (SIM) is employed using gas-liquid chromatography-mass spectrometry. The preparation of the bis(D3-methyloxime), bis-methyl ester of PGE-M containing a tritium tracer in position 2 which was used as internal standard for the SIM method is described. The synthesis of this internal standard includes the biosynthetic conversion of 11-hydroxy-9,15-diketoprostanoic acid to PGE-M by the rabbit. The intra-assay coefficient of variation of this SIM method ranged between 4.0 to 6.7 percent. The recovery of authentic, underivatized PGE-M added to urine was 93 +/- 3% (mean +/- SEM, n=17). The levels of PGE-M excreted in urine were higher (p less than 0.001) in males than in females (15.2 +/- 1.9 mug/24 hours (n=24) and 3.3 +/- 0.3 mug/24 hours (n=17), respectively. These levels were in close agreement with values published previously. No significant difference in excretion of PGE-M between the sexes was observed in the pre-pubertal age-grou (male: 2.9 +/- 0.8 mug/24 hours, n=5; female: 3.1 +/- 0.9 mug/24 hours, n=5) or in the age-group of 45-80 years (male: 9.3 +/- 1.1 mug/24 hours, n=21; female: 7.3 +/- 0.9 mug/24 hours, n=12). The amount of PGE-M excreted decreased significantly after administration of indomethacin or acetyl salicylic acid in therapeutic doses. The concomitant reduction of the urinary excretion of PGE-M (68 to 85% decrease) and prostaglandin E (73 to 100% decrease) after indomethacin treatment in each case (n=8) is evidence that a diminished urinary PGE-M output reflects a decrease in prostaglandin E biosynthesis.