The present study has demonstrated that the mystacial vibrissae of the mouse began to develop at about 12 days of gestation on two plates of thickened ectoderm called the 'whisker pads' which were located on either side of the snout above the margin of the upper lip. Each whisker pad was traversed by five rostrocaudal skin ridges. The individual vibrissae developed along the ridges in a caudorostral sequence. Four new sub-stages of vibrissa follicle development which occurred prior to Stage 1 of Davidson & Hardy (1952) were described. The first of these, Stage A, was the formation of a small nerve plexus under the skin ridge. Stage B was then characterized by the formation of a dermal condensation above the nerve plexus. The epithelium over the dermal condensation began to thicken at Stage C and grow down into the dermal condensation at Stage D. The early morphogenesis of the mystacial vibrissa follicles of the mouse was compared to that of teeth and mammary glands. The possibility of nerve involvement in determining the pattern of follicle array on the snout was discussed. The sequence of the morphological changes in the dermal and epidermal components of the early follicles was related to the present knowledge of epithelial-mesenchymal interactions which occur during this phase of follicle development.