A guanosine-5'-hydrazide can entrap biologically interesting molecules such as acyclovir, vitamin C, and vancomycin into its hydrogel network. Controlled release of these molecules was monitored by 1H NMR spectroscopy. The hydrazide may potentially form mixed G-G quartets with analogous compounds containing a guanine group. 1H NMR spectroscopy was used to study the inclusion of various guanine derivatives into the hydrogel. The structural selectivity was found to depend strongly on both the shape and the charge of the additive and may arise from the strong cohesion of the supramolecular architecture of the gel and the resulting resistance to perturbation by foreign bodies. Hydrogels thus offer a promising medium for highly selective, controlled release of bioactive substances.