Functional studies of the development of the corpus callosum in the cat have shown that an intact callosum during postnatal month 1 is necessary for normal visual development. In vivo tracing techniques have not provided enough information on corpus callosum connectivity to fully evaluate the evidence for a morphological mechanism for the functional effects of neonatal callosum section. However, lipophilic in vitro membrane tracers permit a more detailed search for such evidence because the entire limit of many cells can be labeled simultaneously. To investigate the morphological basis for the observed functional results in cats, the corpus callosum was labeled in vitro with the carbocyanine dye, DiI. Crystals of DiI were placed in the midsagittal callosum in tissue from 2 to 277-day-old cats. Tissue was coronally sectioned 3-22 months later. Sections were photographed and reconstructed to show the overall distribution of corpus callosum projections, as well as the locations of individual corpus callosum axons and their presumed terminals. The distribution of corpus callosum projections, examined in cortical areas 17-19, 7, and posterior medial lateral suprasylvian cortex, changes significantly during development. During postnatal week 1, callosal axons extend throughout these cortical areas to layer I. Numerous varicosities on callosal axons are located en passant and at axon terminals in layer I. During postnatal week 2, the density of callosal projections is reduced in all cortical areas, although many axons still extend to layer I. By postnatal month 2, the callosal axons extending to layer I are predominantly near the border with adjacent cortical areas; in the nonborder regions of these areas, many axons extend to layer VI while a much smaller number of axons extend to layers II-V. By postnatal month 3, the callosal projections to supragranular layers are almost exclusively restricted to cytoarchitectonic border regions; in the remaining regions, including medial area 17, there are occasional axons extending to the supragranular layers and only a moderate number of axons extending to infragranular layers. Thus, a substantial number of elaborately formed transitory corpus callosum axons, distributed throughout visual cortex, exist for several weeks during postnatal development; in area 17, these axons are found in central through peripheral visual field representations. The transitory callosal axons appear to have axon terminals in layer I as well as en passant terminals while extending through layers II-VI. If some of these terminals were to form synapses, there would be extensive opportunities for the corpus callosum to provide input to layers I-VI throughout visual cortex during the period of development in which cortical microcircuitry is being established.