Abstract: Reversibly gel-forming compositions for sustained delivery of bio-affecting substances are disclosed. The compositions exhibit significant changes in viscosity in response to changes in pH. The compositions contain relatively low concentrations of a stable combination of at least one pH-responsive gelling polymer and at least one other thermally nonresponsive polymer. The compositions are preferably formulated to include one or more therapeutic or diagnostic agents for administration as a liquid that will gel in situ or for topical application as a pre-formed gel.