This study describes the overall arrangement of geniculocortical input representing the system of cortical ocular dominance bands in layer IV of striate cortex in the adult cat. The pattern of ocular dominance bands was revealed by transneuronal transport of the intraocularly injected tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Our data indicate that this procedure does not damage the retina and that it results in relatively uniform uptake and transport of the tracer. Using previously published techniques (Olavarria and Van Sluyters, 1983, 1985), both cortical hemispheres of each cat were unfolded, flattened and tangentially sectioned. Analysis of the WGA-HRP labeling patterns in these sections revealed a relatively continuous network of irregularly branching bands in layer IV of area 17 in both hemispheres. Because of a systematic difference in the level of interband labeling, ocular dominance bands appear less distinct in the hemisphere contralateral to the injected eye. There is also a tendency for interband labeling to be greater in cortical regions that represent the more peripheral aspects of the binocular portion of the visual field. The width of an individual ocular dominance band in the cat fluctuates, so that it appears to be made up of a series of uniformly sized, roughly circular beads of label. The diameter of these beads averages 667 micron, and preliminary counts indicate that there are 650-675 beads in each striate cortex. Contrary to earlier suggestions, in 4 out of 6 hemispheres analyzed quantitatively there was no tendency for ocular dominance bands to be oriented along a preferred axis in cat striate cortex, including an axis orthogonal to the border between areas 17 and 18. Ocular dominance bands in area 18 appear to be broader than those in area 17, and they seem to have a greater tendency to be oriented orthogonal to the 17/18 border than those in area 17. Compared with the ocular dominance pattern in monkey striate cortex, the ocular dominance pattern in the cat is much less regular. In general, cat ocular dominance bands appear to fluctuate more in width, to change direction more often, and to be less likely to run orthogonal to the 17/18 border. The greater regularity of the primate ocular dominance pattern may be related to differences in the way in which the visual hemifield is mapped onto the striate cortex in these 2 species.