During the 1991 Gulf War, several U.S. fighting vehicles were hit by friendly-fire munitions containing DU. Soldiers in or near the vehicles were exposed to airborne DU, and some were left with embedded fragments of DU in their tissues. Evaluation of the radiological and chemical risks from these exposures to DU requires estimation of the rate of mobilization of U from the sites of entry into the body to blood and the time-dependent tissue concentrations of the mobilized U. The mobilization and distribution of U cannot be measured directly but must be inferred from the rate of urinary excretion of U, using biokinetic models.