The growth of hair follicles has been traced in guinea pigs with the use of tritiated thymidine injected intradermally. Twenty-four hours after plucking club hairs from resting follicles, desoxyribonucleic acid (DNA) synthesis occurs in the deeper portions of the hair germ and at its periphery. When in anagen VI the follicle is fully formed, DNA synthesis is limited to the hair matrix, below Auber's critical level.