Many polymer gels undergo a volume phase transition in electric fields. We report here the synthesis of a polyelectrolyte gel that incorporates magnetic particles of iron oxide into the polymer network; these particles couple the gel volume transition to the NMR relaxation times of the surrounding water. We find that the water proton relaxation rates (T2(-1)) in aqueous suspensions of particles of the magnetic gel increase significantly in an electric field. Such changes can also be induced by the hyperpolarization of red blood cells in the suspension, presumably as a result of the electric fields at the cell membrane surfaces. These gels may play a part in magnetic resonance imaging analogous to that of voltage-sensitive dyes in optical imaging.