1. To understand the structural basis of the different types of interhemispheric synchronizations described in the preceding paper, we made sections of the corpus callosum and lesions of extrastriate cortex. We measured the effects of such operations on the frequency of encounter, width and strength of T, C, and H peaks in cross-correlation histograms computed from single-unit and multiunit recordings from areas 17-18 of opposite cortical hemispheres in the cat. 2. Sectioning of the corpus callosum led to a complete abolition of T and C couplings and a strong reduction of encounter rate and strength of H coupling. A section limited to the posterior half of the corpus callosum abolished T and C couplings completely. This suggests that T and C couplings are mediated by the direct reciprocal connections between visual cortical areas circulating through the posterior part of the corpus callosum. 3. The encounter rate of H peaks depended on the extent of the callosal cut. Larger lesions gave a more pronounced reduction of the number of H peaks. From this observation we conclude that H peaks are at least partially mediated by polysynaptic pathways involving widely distributed cortical regions. 4. Extensive lesions of extrastriate cortex were made by aspiration of the gray matter or injections of ibotenic acid. These lesions removed the direct inputs from cortical areas sending ipsilateral as well as contralateral inputs to the area 17-18 border region. Encounter rate and coupling strength of C and H peaks were decreased, whereas little effect was observed on T peaks. 5. These results demonstrate that all types of interhemispheric synchronization are mediated by corticocortical connections and that T and C peaks are generated by reciprocal connections between areas 17 and 18 of each hemisphere. Feedback connections play a role in mediating or facilitating the C and H types of interhemispheric synchronization.