AbstractThe pattern of neocortical projections to the mesencephalon was studied in the marsupial phalanger (Trichosurus vulpecula). Lesions were placed in selected neocortical areas in 29 adult phalangers. After a one to two week survival time, the animals were perfused, the brains and spinal cords removed and processed by either the Nauta‐Gygax technique or the method of Fink and Heimer. The cytoarchictecture of the neocortex was studied also and those results pertaining to the ablated cortical areas are reported. The cortex just rostral to the orbital sulcus (motor area for the head and neck) contains numerous dark staining pyramidal neurons which measure up to 35μ at their widest dimension. Lesions within this region produce fiber degeneration within the ventral tegmental area, the nucleus linearis, the periaqueductal gray, the deeper layers of the superior colliculus and the lateral tegmental area. The frontal pole cortex contains very few large pyramidal neurons. Ablations of this cortex result in a similar pattern of fiber degeneration, although few, if any, degenerating fibers are present within either the superior colliculus or the lateral tegmentum.Cytoarchitecturally the cortex just caudal to the orbital sulcus (forelimb motor and some sensory cortex) is strikingly similar to the immediate preobital region. However, destruction of this region produces fiber degeneration within the nucleus of Darkschewitsch, the nucleus of the medial longitudinal fasciculus, the periaqueductal gray, the lateral and deep tegmental areas, the caudal pretectal nucleus and the red nucleus. The motor‐sensory hindlimb area also contains numerous large pyramidal neurons, but the internal granular layer is somewhat better developed than it is in the bulk of the forelimb region. Lesions within the motor‐sensory area for the hindlimb produce fiber degeneration within the same general regions as do ablations of the forelimb cortex, although more degenerating fibers are present within the red nucleus and fewer within the caudal pretectal nucleus.The somatic sensory face cortex (ventral and lateral to the motor‐sensory limb areas) is characterized by a particularly well developed internal granular layer. Lesions of this area produce axonal degeneration in both superficial and deep layers of the superior colliculus. The striate and peristriate areas can be distinguished from one another by the development of the internal granular layer. Lesions in these areas result in fiber degeneration which is limited to the superficial layers of the superior colliculus.The neocortex of the phalanger temporal pole cannot be distinguished by the development of any particular layer. Indirect evidence suggests that this cortical region projects to the nucleus of the inferior colliculus as well as to the deeper layers of the superior colliculus.