This study investigates the influence of various lead geometry on intracardial signals like the monophasic action potential (MAP) to optimize the geometry of implantable MAP leads. The experimental results were compared with a field theoretical approach to the origin of MAP from the transmembrane potential (TAP). During the experiments several lead geometries (tip surface: 1.3 to 12 mm2; tip-ring distance: 0.8 mm to 25 cm; ring surface: 1.8 mm2 to 40 mm2) were investigated in endo- and epicardial positions in 12 dogs (17 +/- 9 kg). The electrodes were fixed passively (tines) or actively (screws). MAP was recorded during several interventions and correlated with MAP measured using an Ag-AgCl MAP catheter. The experimental results showed that small tips provided high MAP amplitudes with less pressure. No difference was observed using active and passive fixations. A tip-ring distance smaller than 5 mm with a ring surface smaller than the tip (< 5 mm2) avoided artifacts in the repolarization course. For the theoretical approach the quasistatic, anisotropic bidomain model was calculated in small unity volumes Vi where the TAP phi mi was constant and represented by the current density Ji. Two solutions for electrode positions at and outside the heart were achieved. By superposition of each solution phi ei the summed potential at the electrode position was calculated. The theoretical findings show in good correlation with the experimental results that a larger distance than 10 mm leads to distortions in repolarization course by signals proportional to phi out.