The literature concerning the pain pathways in the central nervous system and their surgical interruption for the alleviation of pain has been extensively reviewed.1The present communication adds direct observations which concern the localization of the pain fibers from the body at the level of the inferior olive and confirmatory data on the course of the spinothalamic tract through the brain stem. The technical procedure of interrupting the spinothalamic tract in the medulla, resulting in relief of high, intractable pain on the opposite side of the body, has been described in an earlier report.2On the basis of the sensory changes noted during the operation, it was concluded that a topical arrangement of fibers is present within the spinothalamic tract, so that at the level of the inferior olive fibers belonging to the lower dermatomes of the body lie more superficially while those of the upper ones occupy