A new specially designed analytical function approximating the intracellular action potentials (ICAPs) for calculation of the extracellular potentials (ECAPs) at various radial and axial distances from the active fibre is proposed. 4-Aminopyridine (4-AP) was used to obtain ICAPs with a prolonged repolarization phase in order to investigate the influence of changes in ICAP shape on the ECAPs. From the experimentally recorded ICAPs before and after treatment of frog skeletal muscle fibres with 4-AP, approximated by the new function, the ECAPs were calculated applying the linesource model in a finite fibre. Using this function allowed calculation of the ECAPs at distances not accessible for the experimental recordings. The total ionic current (I i) during the action potential was calculated using the cable equation. Our results showed that the ratio of the first positive to the negative phases of the ECAPs of treated fibres increased at large radial distances (3000 μm and more) and the terminal positive phase was asymmetric with an abrupt initial deflection followed by a slow inverse deflection. The calculated ECAPs at various axial distances from the fibre end (cylindrical and conical part) and at radial distances from the fibre membrane ranging from 0 to 5000 μm, corresponded in shape to the experimentally recorded potentials of untreated and 4-AP-treated muscle fibres.