Surgical interruption of the pain pathways in the mesencephalon demands an accurate knowledge of the position of the spinothalamic and secondary trigeminal tracts. While considerable data have been accumulated on this point, there is practically no information on the topical localization of the spinothalamic tracts in the mesencephalon. PRESENT INVESTIGATION In an attempt to determine the arrangement of the spinothalamic and secondary trigeminal tracts in the mesencephalon the following operations were performed on 4 monkeys (Macaca mulatta). Experiment 1: A lesion of the spinal nucleus of the trigeminal nerve. Experiment 2: A midline myelotomy from the twelfth thoracic to the first lumbar segment. Experiment 3: A midline myelotomy from the fourth cervical to the sixth cervical spinal segment. Experiment 4: A midline myelotomy from the fifth lumbar to the seventh lumbar segment. The animals were killed two to three weeks after operation and the brains placed in Müller's fluid. The