At the currently administered clinical doses, paramagnetic metal chelate complexes presently used as MR contrast enhancement agents appear to be relatively nontoxic. Solution thermodynamic, solubility, and selectivity studies, based on a number of gadolinium chelate complexes, indicate that very little gadolinium is released in vivo, and the small amounts that do remain are available for excretion, albeit slowly. Although the mechanism of metal release from manganese-based chelate complexes is not well understood, any released manganese is likely to be quickly and efficiently cleared through the liver. Although MRI contrast agents are unlikely to be administered repeatedly in patients, which could result in accumulation of metal ion, the long-term effects of such potential deposition have yet to be demonstrated.