Neonatal damage to a row of mystacial vibrissae in the mouse causes cytoarchitectonic alterations in the contralateral SmI barrel Cortex. The region for the appropriate row of barrels develops as a smaller homogeneous zone while barrels in adjacent rows are expanded. To investigate the effects of this phenomenon on the morphology of individual neurons, adult mice in which Row-C vibrissae (the middle row) had been cauterized on days 1--5 following birth were processes by the Golgi-Cox method. All neurons in layer IV of the Row-C zones, of the Row-C barrels of a control hemisphere, and some neurons in the adjacent enlarged Row-B barrels were measured with a computer-assisted microscope. Their location with respect to cytoarchitectonic boundaries was determined from a Nissl counterstain. Data from 239 cells are presented. For each cell, measures of dendritic length and branching were obtained. The orientation of the dendritic trees with respect to the barrel sides was also measured. The measures of dendritic lengths and branching did not show any differences between control and experimental animals or between animals damaged on different days. Measures of orientation did show changes related to the age at the time of damage. In animals damaged on postnatal day (PND)-3 or earlier, many cells in the Row-C zone were observed with dendrites orienting toward the adjacent Rows-B or -D. "Putative" Row-C cells in the expanded parts of Rows-B and -D were strongly oriented toward barrels in those rows. These results suggest that dendritic length and branching may be determined intrinsically but that the orientation of the dendritic trees appears to be strongly influenced by the pattern of extrinsic afferent inputs from the thalamus. In the case of the whisker-damaged animals, the orientation of the Row-C neuron dendritic trees toward the "functional" thalamocortical inputs in Rows-B and -D contributes strongly to the resultant cytoarchitectonic changes. The implications of these results for normal developmental processes and their relationship to functional studies of the cortex are considered.