The electrical field generated by activity in a bundle of fibers can be viewed as arising from sources distributed throughout the cross-section. A model study is presented in which the source density is calculated at various depths in a circular, cylindrical bundle of cardiac tissue using expressions derived from a bidomain model. The results show that unlike the equivalent source density of a single fiber which is uniformly distributed in the cross-section, the source density distribution in the bundle is nonuniform. For large bundles (>4 mm), the magnitude of the source strength is greatest near the axis and decreases sharply within 250 mu m of the surface.<<ETX>>