Nine children with Down syndrome were compared to two groups of nonretarded children, one similar in CA, the other a chronologically younger group of similar MA. The event-related brain potential (ERP) and reaction time (RT) results indicated that children with Down syndrome process some types of auditory information more slowly than do MA- or CA-matched nonretarded children. They were found to differ from nonretarded children in the scalp distribution of amplitudes of certain ERP components. Finally, the speed of processing and amplitude differences that were found could not be explained on the basis that children with Down syndrome are simply maturationally delayed in their cognitive abilities. The possible relation of these neuropsychological (ERPs) and performance (RT) differences to pathological changes in the hippocampus were discussed.