8-iso-Prostaglandin F2alpha (8-iso-PGF2alpha) is a product of free radical-catalyzed peroxidation of arachidonic acid. Measurement of its urinary excretion has been proposed as an index of oxidative status in vivo. A stable isotope dilution method for its quantification by gas chromatography-electron capture chemical ionization mass spectrometry is described. Sample cleanup required the combined use of high-performance liquid chromatography and thin-layer chromatography. The inter-assay R.S.D. in two separate determinations was 1.6 (n=4) and 2.3% (n=4). The accuracy of the assay was evaluated through recovery experiments. The equation of the regression plot correlating the amounts added and recovered was y=0.91x-0.31, r=0.9916 (n=12). The pair of fragment ions ([M-181]-) at m/z 569 and m/z 573 was monitored for quantification. The mean 8-iso-PGF2alpha excretion rate was 528 +/- 127 (S.D.) ng per day in five male volunteers and 730 +/- 305 ng per day in six females. Intake of 80 mg of lycopene per day by eleven volunteers for four weeks resulted in a non-significant reduction of 8-iso-PGF2alpha excretion.