In mice from 6 strains bred for different patterns of mystacial vibrissae, we studied the consequences of the presence of supernumerary whiskers for the sensory innervation of the vibrissal follicles and their cortical representation in the barrel field. The parameters were number of axons innervating individual vibrissal follicles, tangential area of individual barrels, and thickness of the layers in the barrel cortex. These parameters are highly constant for animals within a strain but may differ greatly between strains. For all strains, the innervation of a follicle depends on its position on the whiskerpad, the highest innervation density being at the posterolateral corner. This matches the wave of development that travels over this part of the face during embryogenesis. Although large differences exist between strains in the number of axons innervating the whiskerpad, the relative number of axons innervating the "standard" follicles of 1 row is remarkably constant. The thickness of the barrel cortex increases from posteromedial to anterolateral for all strains. This increase is due to variations in thickness of the cortical output layers (II and III, V and VI). For individual barrel-follicle pairs, we calculated the ratio between cortical barrel area and axon number. The major findings were that supernumerary follicles are innervated and, given a threshold number of axons, represented by barrels; barrel area per peripheral axon differs between follicles within a row and is highest for the supernumerary elements; and for each strain there is a direct, linear correlation between axon number and barrel size, which differs significantly among certain, but not all, strains. The data allowed us tentatively to define some of the factors that play a role in the formation of brain maps and pointed to the existence of major genetic differences between mouse strains with respect to these factors.