Gadolite, a positive oral contrast agent for magnetic resonance imaging (MRT) that is currently in clinical trials, is based on a gadolinium(III)-modified NaY zeolite. Studies relevant to the in vivo stability and efficacy of GaNaY as an MRI brightening agent are detailed in this paper, The stability of GdY at low pH as well as in the presence of Na+, K+, Mg2+, and Ca2+ at concentrations relevant to the gastrointestinal tract has been evaluated. The relaxivities (1/T-1 vs [Gd]) of aqueous GdY suspensions were measured at 40 MHz and room temperature. Both the thermal history and gadolinium loading in the zeolite were found to have a profound effect on the relaxivity.