The skin and its appendages provide an excellent opportunity for studying regeneration of normal tissue. As a result of an accidental hammer blow to the thumb nail, the author had the opportunity to study regeneration of the damaged nail in great detail. For several weeks after the injury, there was a complete cessation of nail growth. The first nail which formed was markedly reduced in thickness, a result of pressure from the hematoma. When the full thickness nail finally grew out, it showed phases of increased thickness and then thinness lasting several months before the normal nail appeared. A comparison of these findings with similar observations on the hair suggests that the inherent metabolic activity of a tissue may affect the mechanisms which regulate regeneration following injury.