In submammalian vertebrates the optic tectum, a structure homologous with the mammalian superior colliculus, is the primary way-station of the fibers of the optic tract. As such, it is assumed to be of central significance in the coordination of visually-guided behavior (Huber and Crosby, '33; Ariens Kappers, Huber and Crosby, '36) . Ascending the evolutionary scale, the superior colliculus of mammals receives fewer and fewer fibers from the optic tract, as an increasing proportion of these fibers terminate in the lateral geniculate nucleus (Polyak, '57). In spite of this apparent comparative decline, the size and complexity of the mammalian superior colliculus (Huber and Crosby, '43) suggests that it is by no means a vestigial structure of little functional importance. The fiber projections of the superior colliculus have been investigated by means of several staining techniques (e.g., Golgi, Weigert, Marchi) in a variety of mammalian species, including man (Tsai, '25, in opossum; Held, 1890, 1892, 1893, Papez and Freeman, '30, in rat; Szent6gothai-Schimert, '41, in mouse; Cajal, 1896, Muenzer and Wiener, '02, Lewandowsky, '04, De Lange, '10, Nishikawa, '23, in rabbit; Held, 1890, 1892, 1893, Bechterew, 1897, Tschermak, 1898, Redlich, 1899, Probst, 1900, Collier and Buzzard, '01, Crosby and Henderson, '48, in monkey; and Held, 1890, 1892, 1893, Bechterew, 1897, Collier and Buzzard, '01, Tsuchida, '06, in man). Most of these investigators were concerned largely, or exclusively, with the descending fiber projections of the superior colliculus, and their data indicate numerous discrepancies. The present study was undertaken in an attempt to determine both the descending and ascending projections of the superior colliculus in the cat by means of the Nauta-Gygax technique, following localized stereotaxic lesions in the superior colliculus.