The advantage of using sphenoidal (SE) over antero-temporal electrodes (ATE) remains controversial among epileptologists. Yet, in a recently published study of 17 patients with seizures of antero-temporal origin (Kanner et al., 1995), we demonstrated that SE placed under fluoroscopic guidance (FPSE), in order to insure that their recording tips are positioned immediately below the foramen ovale (FO), yielded a significant advantage over SE placed with the standard blind method of insertion (BPSE), in both interictal and ictal recordings. This study was done to test the following hypothesis: FPSE advantage over BPSE and ATE resides in the recording of epileptiform activity with a restricted electric field. We compared spike voltages at FPSE, BPSE and ATE in sets of 5 randomly selected spikes per interictal focus, recorded in the course of separate monitoring studies with BPSE and FPSE. We represented the voltage differences as ratios, VATE/FPSE and VATE/BPSE and calculated a mean ratio for each spike set. The spikes' voltage was almost identical at BPSE and at ATE (mean VATE/BPSE = 0.94), while it was significantly higher at FPSE than at ATE (mean VATE/FPSE = 0.66; P < 0.001, t test). A significantly narrower electric field contour was found among interictal foci in which FPSE yielded additional data during interictal (P < 0.001) and ictal (P = 0.016) recordings. Conversely, VATE/FPSE did not differ from VATE/BPSE among interictal foci where FPSE failed to yield any advantage over BPSE in either interictal (P = 0.240), or ictal (P = 0.311) recordings. These findings prove that SE yield additional localizing data when recording epileptiform activity with a restricted field, provided that its recording tip is positioned below the FO. When distant from FO, SE can be expected to yield comparable data to that obtained with ATE.