Hectorite type clays modified with gadolinium(III) were prepared by ion-exchange of natural (SHCa) hectorite and synthetic (SLH) hectorite. Results show that Gd(III) content in the clay increases with increasing gadolinium(III) in the exchange solution within the range between 0 and 25% of the Gd/clay weight ratio. Studies relevant to the in vivo stability and efficacy of Gd(III) ion-exchanged hectorite as an oral MRI contrast agent are detailed. The stability of Gd(III) ion-exchanged hectorite at low pH as well as in the presence of sodium ion at concentrations relevant to the gastrointestinal tract has been evaluated. The room-temperature relaxivities (1/T-1 vs [Gd]) of aqueous Gd(III)-exchanged hectorite suspensions at different stirring time, concentration, and pH were measured at 40 MHz and room-temperature. The type of clay and the gadolinium loading as well as the amount of clay in suspension and the stirring time were found to have a profound effect on the relaxivity.