An animal model was developed in order to estimate a potential in vivo dechelation of gadopentetate dimeglumine (Gd-DTPA; CAS 86050-77-3) and to provide information concerning the chemical form of long-term gadolinium not excreted. Excretion kinetics and biodistribution of 153Gd-DTPA and Gd-[14C]DTPA were studied in 2 groups of rats following the intravenous injection of 0.5 mmol/kg Gd-DTPA. Differences in the elimination and biodistribution profiles between the quantities of 153Gd and 14C radioactivity were taken as evidence for dechelation of Gd-DTPA within the body leading to a release of Gd ions. Statistically significant differences were observed in excretion and whole-body retention indicating an in vivo dechelation of less than 1% of the injected dose. 30% of the released Gd ions were excreted via urine and feces during the 21-day time interval. 21 days p.i., 70% of the released Gd ions remained in the body producing a significant 153Gd excess in liver, spleen, and the bones representing the principal target organs of released Gd ions after intravenous injection of Gd-DTPA. More than 96% of the released Gd ions found in the body 21 days p.i. were deposited in the bones. Liver and spleen showed minor contributions. In any case, the chemical form of the gadolinium present in liver, spleen, and the bones could predominantly be assigned to translocated Gd ions resulting from dechelation of Gd-DTPA.