The destruction of a row—row C—of follicles of the mystacial vibrissae in 106 mice during early postnatal life caused alterations of the barrels in the contralateral first somatosensory area of the cortex that corresponded to the lesioned sites. It is to be noted that barrel neurones are placed three synapses away from the periphery. Destruction of follicles on postnatal day 0 to 6 inclusive produced alterations of corresponding barrels in the cerebral hemispheres of the animals killed when 12 days old. This suggests that in the development of the somatosensory system of the mouse there is a critical period, during which peripheral insult leads to abnormal cytoarchitectonic cortical patterns. These patterns varied and were categorised, a given type being predominant for a given day of lesion. After lesions on postnatal day 0, a narrow, elongated barrel-like territory occupies the lesion-related domain. After lesions on postnatal day 1, this domain is reduced to an elongated island of cells in disarray, or is absent (barrelless territory). Lesions on postnatal day 2 caused a pattern that is transitional between the barrel less territory and the poorly defined barrels, which are predominant following lesions on postnatal day 3. Lesions on days 4, 5 or 6 produced (abnormally) sharply defined barrels. Whereas all previous modifications were confined to the hemisphere contralateral to the lesion, the last lesion-group displayed enlarged septa in both hemispheres. Wherever the lesion-related domain occupies an area smaller than the normal barrel row which it replaces, the adjacent rows ‘fill in’. Surface area measurements and statistical analysis allowed us to make the following point: (i) After lesions from day 0 to day 3 the surface area of the cortical lesion-related domain was reduced; it was only after lesions on days 2 and 3 that the increase in area of the neighbouring rows offset this reduction. (ii) Alterations due to lesions from birth to day 6 do not follow a gradual progression toward normal, as described by others. Indeed, the greatest reduction of the lesion-related domain was found following lesions on day 1 and not day 0. (iii) Following lesions on days 4, 5 or 6, an increase in area of the lesion-related septa corresponded to a reduction in area of the lesion-related barrels. Tentative interpretations of our findings include collateral sprouting of intact peripheral sensory nerve fibres, degeneration of neurones whose peripheral, sensory terminals have been destroyed, peripheral and central regeneration of lesioned Gasserian neurones, a modified balance in the competition of the terminals for postsynaptic space, atrophy of neurones that receive a decreasing number of terminals or impulses, loss of the remodelling capacity of a centre after its invasion by synapses, stabilisation of early exuberantly distributed callosal terminals, and an altered parcellation of the thalamocortical fibres from the onset of their ingrowth into the cortex.