The normal cytoarchitectonic pattern of barrels in layer IV of mouse SmI face cortex is altered by early damage to the mystacial vibrissae (Van der Loos and Woolsey, '73). In the present study, the middle row of vibrissae (row-C) on one side of the face in groups of Swiss mice was cauterized on the day of birth (postnatal day [PND] -1)or on PND's - 2, 3, 4, 5, 7, 10, 12, 15, 20 and 30; littermates in each group served as controls. All animals were perfused on PND-60 and the brains sectioned parallel to SmI layer IV. For each specimen, the posteromedial barrel subfields (PMBSF) of the two hemispheres were reconstructed with a camera lucida and the cross-sectional areas of individual barrels measured using a small computer. The findings are: (1) The hemispheres ipsilateral to the vibrissal damage are quantitatively indistinguishable from the littermate controls indicating that the ipsilateral hemispheres in lesioned animals can serve as controls for observations of the type reported in this paper. (2) There are no consistent differences in the cross-sectional areas of the PMBSF's as a whole in the hemispheres ipsi- and contralateral to the peripheral damage, suggesting that there is no net loss of cortex as a result of the lesions. (3) The contralateral row-C barrels are reduced in size. Expressed as a percentage of normal values; row-C is reduced to 17% for animals lesioned on PND-1, 16% on PND-2, 38% on PND-3, 52% on PND-4 and 79% on PND-5; on PND-7 and later the cross-sectional areas of row-C barrels are normal. This implies that the barrel field of the SmI face cortex becomes progressively refractory to the effects of peripheral damage during the first postnatal week and in the period prior to PND-6, an intact periphery is necessary for normal cortical development. (4) In every case, the decreased cross-sectional area of row-C is accompanied by precisely increased cross-sectional areas of the barrels in adjacent rows-B and D. in the case of the restricted peripheral damage which we produced, there is a "compensation" in the contralateral hemisphere, which can be correlated with patterns of the specific thalamocortical projections.