A study was made of corticofugal projections to the lower brain stem and spinal cord in the tree shrew (Tupaia glis). Degeneration resulting from selective ablations of the motor, sensory and frontal cortex in ten animals was studied by the Nauta technique. Following ablations of motor and sensory cortex degeneration was found bilaterally: (1) throughout the rostrocaudal extent of the lateral reticular formation, (2) in all trigeminal sensory nuclei and, (3) in the dorsal column nuclei (contralateral predominance). All three cortical areas projected fibers to the ipsilateral pontine nuclei and bilaterally to the medial reticular formation (ipsilateral predominance). Fibers from the motor cortex were distributed throughout the medial reticular formation; fibers from sensory and frontal cortex were distributed to selective regions of the medial reticular formation. The majority of corticospinal fibers decussated and descended to lower cervical (frontal fibers) and lower thoracic (motor and sensory fibers) levels. The small uncrossed component descended in the ipsilateral dorsal funiculus only throughout cervical segments. Corticospinal degeneration terminated primarily in the internal basilar region (medial half of lamina VI), medial aspect of the neck (lamina V) and, to a lesser extent, in the head (laminae III and IV) of the dorsal horn. Relatively few fibers projected to the zona intermedia (lamina VII) and no terminations were present in the motor nuclei (lamina IX) of the spinal nerves.