The invention provides methods and compositions for molecular resonance imaging of membrane potential. The compositions comprise an indicator moiety attached through an optional linker to a macrocycle component that binds a lanthanide. The indicator moiety changes conformation in response to alterations in membrane potential and alters the interaction of the bound lanthanide with water. This alteration produces a detectable change in MRI signal , notably the T.sub.1 value. A preferred indicator moiety is spiropyran as it exhibits membrane-potential driven interconversions between the closed-ring spiropyran conformer and the extended merocyanine conformer. A preferred macrocycle is DO3A, known to bind gadolinium and an established MRI contrast agent. The methods of the invention comprise in vivo and in vitro uses of compositions of the invention to obtain MRI images that provide information about membrane potential.