a magnetically responsive composition for the modulated , sustained administration of a biologically active substance , the composition being in the form of a body sized and shaped for placement in the environment of use , the environment including an aqueous fluid , the body comprising , in admixture , a first phase comprising a biocompatible , plastically deformable , polymeric matrix , the polymeric matrix being insoluble in the environment of use , a second phase having a biologically active substance distributed throughout the matrix to be released to the aqueous fluid outside the matrix , and a third phase comprising a magnetically responsive substance encapsulated within the matrix so that , upon exposure of the body to an oscillating magnetic field , the rate of release of the biologically active substance to the aqueous fluid is increased .