Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two pyrrole units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been evaluated by a series of different approximations and a large Gaussian basis set. An ab initio model of magnetic field induced current density was obtained for four isomeric systems, showing that strong diamagnetic flow takes place within the pi electrons. The pi currents are responsible for exalted magnetic anisotropy and proton deshielding. The theoretical findings are used to assess a diatropicity scale for these molecules. (C) 2003 American Institute of Physics.