We studied sensory organ reinnervation after nerve transection in the mouse whisker-to-barrel pathway. In one set of adult mice, we determined at light microscopy level the number of fibres reaching the caudal whisker follicles 5, 15, 20, 60, 100 days and 1 year after transection of the sensory nerve of row C. Regenerated fibres were first detected 15 days post lesionem (p.l.) and myelin first observed at 20 days. Between 60 and 100 days, the number of fibres stayed at approximately 80% of the values obtained in control animals. At that time, myelinated fibres reached only 58% of their number in controls. At the electron microscopy level, these fibres differ from control ones by a smaller fibre diameter. The innervation of follicles of adjacent rows was not modified, indicating that follicular reinnervation is row specific. We checked this feature by injecting in another set of mice the denervated follicles and the adjacent ones with distinct retrograde tracers 45 days and 1 year after nerve transection. The percentage of double-labelled neurons in the Gasserian ganglion did not increase in experimental animals. This confirms the absence of colonization of intact follicles by regenerating fibres and indicates that reinnervation of the whisker follicles takes place by regeneration of the degenerated axons without collateral reinnervation. The companion paper describes the pattern of activation of the barrel cortex relative to the present findings.