A sequence of seven low-amplitude (nanovolt) potentials that occur in the initial 10 msec following click signals can be recorded from scalp electrodes in human subjects using computer averaging techniques. The potentials, termed auditory brain stem responses, are thought to be the far-field reflection of electrical events originating in the auditory pathway during its course through the brain stem. We have studied auditory brain stem responses in a variety of neurological disorders and found them to be of assistance in evaluating the mechanisms of coma, the localization of midbrain and brain stem tumors, the localization of demyelination of the brain stem, and tumors, the localization of demyelination of the brain stem, and the presence of diminished brain stem circulation.