AbstractSections prepared by the Nauta method were used to study the fiber degeneration in the dorsal lateral geniculate nucleus and the pulvinar of the rabbit after lesions in the visual cortex, and sections processed by the Golgi and the Cajal methods provided the material needed to study the normal fiber architecture of these nuclei.Two distinct systems of fibers are recognizable in the dorsal lateral geniculate nucleus and in the pulvinar. One is a transverse system formed by the fiber bundles which run within these nuclei oriented in parallel with the optic tract; some of these bundles mingle with the fibers of the optic tract. The other is a longitudinal system whose fibers course perpendicularly to those of the transverse system. Lesions in the visual cortex produce degeneration within both of these fiber systems, in the geniculate nucleus as well as in the pulvinar. The degenerating transverse fibers are corticopretectal and corticotectal axons, and these send collateral branches into the longitudinal systems of these nuclei.The degenerating longitudinal fibers are of two types. One type consists of fibers which are fine in caliber, give rise to numerous short terminal branches, and degenerate early (undegenerated fibers showing this same morphology are identified in the Golgi preparations). These longitudinal fibers are the collateral branches of the degenerating transverse fibers and at the same time they represent the terminal portions of the fibers which provide the visual cortical input to the geniculate nucleus and the pulvinar. It is noteworthy that within the geniculate nucleus these terminal fibers run along the lines of projection for individual points on the visual field.The second type of degenerating longitudinal fibers is found in the dorsal lateral geniculate nucleus but not in the pulvinar. Its fibers are coarser and degenerate later than the longitudinal fibers described above, and they are probably geniculocortical axons undergoing retrograde degeneration.