The anatomical organization of projections from the microgyric cortex to the contralateral hemisphere through the corpus callosum has been studied in the rat. Microgyria was induced on the first postnatal day by a freezing injury. Once adult, animals received injections of horseradish peroxidase in the cortex contralateral to the lesion. Callosally projecting neurons located in the normally layered cortex adjoining the focal lesion showed a different laminar distribution from that observed in control animals. The highest percentage of callosal neurons was found in layer 6 in lesioned animals, whereas layers 2/3 and 5 gave rise to most of interhemispheric projections in controls. Possible mechanisms leading to the establishment of aberrant callosal connections and potential implications for the human pathology are discussed.