We measured the excretion of a major urinary metabolite of leukotriene (LT) C4, i.e., LTE4, during the infusion of exogenous LTC4 to enable estimation of the rate of entry of endogenous LTC4 into the bloodstream. Four healthy volunteers received 2-h intravenous infusions of vehicle alone and LTC4 at 0.063, 0.32, 1.6, and 2.9 pmol.kg-1.min-1 in random order. Urinary LTE4 was measured before, during, and up to 24 h after the infusions. The fractional elimination of LTE4 was independent of the rate of LTC4 infusion and averaged 4.3 +/- 0.9%. Calculation of the mean rate of entry of LTC4 into the circulation was found to be 0.06 pmol.kg-1.min-1. In addition, we characterized further metabolism of [14C]LTC4. The two major urinary metabolites were the omega- and beta-oxidation products (16-COOH-LTE4 and 14-COOH-LTE3), which accounted for 6-8% of the total infused amount of [14C]LTC4. We conclude that 1) LTC4 is produced at a low rate under physiological circumstances and is rapidly converted in the vasculature to LTE4, 2) changes in the urinary excretion of the latter may reliably reflect short-term changes in the rate of secretion of LTC4, and 3) measurement of the omega- and beta-oxidation products may reflect chronic changes in cysteinyl leukotriene biosynthesis.