The hair growth cycle is generally recognized to comprise phases of growth (anagen), regression (catagen), and rest (telogen). Whereas, heretofore, the hair shedding function has been assumed to be part of the telogen phase, using a laboratory mouse model and newly developed techniques for quantitative collection and spectroscopic determination of shed hair, we found that shedding actually occurs as a distinct phase. Although some shedding occurs throughout the growth cycle, the largest peak is coupled to anagen. Using hair dye and rhodamine labeling we established that the shafts that shed arise during the previous hair cycle. We found that over the cycle the ratio of shed overfur to shed underfur hair shafts varies with the cycle phase and that the shed shaft base is unique morphologically, having a cylindrical shape with scalloped or "nibbled" edges. By electron microscopy the mooring cells of the exogen root show intercellular separation suggesting a proteolytic process in the final shedding step. This is the first report describing a distinct shedding, or exogen, phase of the hair cycle. This study supports the notion that this phase is uniquely controlled and that the final step in the shedding process involves a specific proteolytic step.