An action potential propagating through a two-dimensional sheet of cardiac tissue produces a magnetic held, In the direction of propagation, the intracellular and extracellular current densities are equal and opposite, so the net current is zero. However, because of the unequal anisotropy ratios in the intracellular and extracellular spaces, the component of the current density perpendicular to the direction of propagation does not, in general, vanish. This line of current produces the magnetic held, The amplitude of the magnetic held is zero only if the action potential propagates parallel to or perpendicular to the fiber direction, or if the tissue has equal anisotropy ratios. © 1999 Institute of Electrical and Electronics Engineers.