Evoked potentials from peripheral stimulation were monitored centrally to delineate the descending trigeminal tract in six patients with facial pain. Lesions were made in the areas of the tract where maximal evoked potentials were obtained. Pain was completely relieved, while touch sensation and corneal reflexes were retained. Associated analgesia of the ipsilateral pharynx, tympanic membrane, and external auditory meatus, obtained in five of the six patients, indicates the close proximity of the pain and temperature fibers of cranial nerves VII, IX, and X to those of the descending trigeminal tract. This physiological localization of the tract contributes to the accuracy of descending trigeminal tractotomy while avoiding the severe discomfort resulting from direct stimulation in the conscious patient.