Stable isotopic methods were employed to determine the proportion of ingested folate excreted as the metabolite p-aminobenzoylglutamate (pABG, free and N-acetyl) in urine. Adult male subjects (n = 7) were maintained on a 4.54 mumol/d (2 mg/d) folate saturation regimen. After 7 d, subjects were given a mixed oral dose containing 0.67 mumol (300 micrograms) each of bideuterofolic acid (d2) and tetradeutero-folic acid (d4). Urine was collected for the following 48 h and analyzed for folate and pABG. The extent of deuterium labeling of urinary folate and pABG was determined by gas chromatography-mass spectrometry. Urinary total pABG excretion increased less than twofold as a result of the folate saturation, whereas urinary folate increased over 10-fold. Urinary d2 and d4 folates each contained 14-15% of the respective oral doses of labeled compounds, whereas urinary d2 and d4 pABG comprised only 0.98-1.15% of the labeled doses. Molar ratios (d2/d4) of excreted folate and pABG indicated that there was no in vivo isotopic discrimination between the labeled folates. Urinary pABG accounted for 5.1 +/- 0.6% of total ingested folate, whereas labeled pABG was about 6.7-7.6% of the excretion of labeled compounds (i.e., labeled folate + pABG). This study indicated that pABG is not a major excretory product during folate supplementation, but its relative importance may increase in conditions of reduced folate nutriture.