The specific goals are to determine (1) Response of a uniform three-dimensional cardiac muscle bundle to an electric current (defibrillating) impulse. Extension to include more realistic cardiac muscle geometry in the form of an eccentric spheres heart model. Solutions will be examined under subthreshold and transthreshold conditions. The presence and effect of intercellular (coupling) junctional resistances on induced transmembrane potential will be particularly examined. (2) Response of a single-fiber cardiac strand to a magnetic field stimulating pulse. Extension to a uniform three-dimensional cylindrical bundle of cardiac muscle fibers, and then an investigation of the eccentric spheres heart model. Solutions under subthreshold and transthreshold conditions will be examined. The effect of junctional resistances and other discontinuities in conductivity will be examined.