Impedance changes were measured with scalp electrodes in human subjects during physiologically evoked responses to assess whether such changes could form the basis for imaging of neuronal discharge by applied potential tomography (APT). Conventional methods were employed to measure cortical sensory and visual evoked responses, and the sensory action potential of the ulnar nerve at the elbow. In each case impedance was measured across the appropriate tissue of origin using a high sensitivity four-electrode method. No consistent changes larger than 0.002 or 0.02% (depending on electrode configuration and period of measurement) of the resting impedance were obtained. Changes within these limits appeared to be due to variations in pulse related impedance changes. This suggests that currently available APT devices could not be used to obtain images of neuronal discharge, and research into APT imaging in the adult head might more profitably be directed at imaging the relatively large impedance changes during stroke or spreading depression.