For individuals with severe or profound hearing loss, electrical stimulation of surviving neural elements by a cochlear implant may partly restore a sensation of hearing. Determining the extent of restoration based on behavioral measures may be difficult, particularly when evaluating young children or individuals who have little or no experience with normal hearing. In normal-hearing individuals, an objective measure of sound discrimination may be obtained by studying the mismatch negativity (MMN) component of the auditory evoked potential. The MMN may be evoked by a number of physical differences in acoustic stimuli including duration and pitch. For cochlear implant users, analogous stimulus differences may be produced by changing the length of a stimulus pulse train or by changing the pair of activated electrodes along a multi-electrode implant array. This paper will provide an overview of our current results, comparing evoked response data recorded from both normal-hearing individuals and cochlear implant users. In both normal- hearing individuals and cochlear implant users, MMNs were evoked by differences in stimulus train duration and pitch (or electrode pair activation in cochlear implant users). These findings suggest that the MMN may be a useful method for assessing the discriminability of electrical stimulation patterns produced by a cochlear implant. Eventually, information gained by MMN testing may yield important information for developing rehabilitation programs for the individual user.