The electrophysiological maturation of both peripheral and 'central' (spinal + superspinal) nervous fibers belonging to peroneal and medial nerves was evaluated within the 1st month of extrauterine life. Thirty-five newborns with different gestational ages (15 FT, 20 PT), have been examined through the SEP technique during percutaneous stimulation of median (wrist and index finger) and peroneal (popliteal fossa) nerves. Recording electrodes were placed at Erb's point, lumbar and cervical spine, scalp sites corresponding to the sensory areas for upper and lower limb. Peripheral conduction velocities were correlated with chronological ages but they did not correlate neither with gestational ages nor with birth weightes. Central conduction velocities correlated with gestational ages. The role of maturation was faster for the former than for the latter.