A supramolecular hydrogel based on D-amino acids, which resists hydrolysis catalyzed by proteinase K and offers long-term biostability, exhibits controlled release in vivo, as proved I-131 the pharmacokinetics of encapsulated I-125 tracers and the SPECT imaging of the hydrogel-encapsulated I tracers. As the first in vivo imaging investigation of the drug release properties of the supramolecular hydrogel, isotope encapsulation serves as a valid, useful assay for characterizing the controlled release properties of supramolecular hydrogels in vivo. Our results indicate that supramolecular hydrogels promise new biomaterials for controlled drug release.