The distribution of callosal neurones of the first somatosensory area (S1), projecting to contralateral second somatosensory area (S2), was studied with horseradish peroxidase in adult cats in which the injected S2 area had been deprived of some of its association and callosal input by an earlier lesion of S1 associated with a lesion of contralateral S2, performed on postnatal days 5 to 30. The distribution of callosal cells in S1 of animals subjected to cortical lesions on postnatal day 14 (2 cases) and 30, was in all respects similar to the controls. Callosal neurones were, in fact, selectively distributed only to parts of regions of the forepaw, trunk and hindlimb representation. However, the normally acallosal regions of S1 contained scattered neurones in the animal with cortical lesion inflicted on day 5. The partial filling-in of the regions of S1, normally devoid of callosal neurones, is probably due to preservation to adulthood of some juvenile callosal neurones which otherwise would have been eliminated or would have lost their callosal branch. Previous studies showed, in fact, that in the early postnatal period, callosal neurones are widely distributed through area S1 in kittens. Some interpretations of the findings are discussed.