SUMMARY: Hypophysectomy of, or administration of thyroxine to, male or female rats advances the activity of hair follicles in the ensuing wave of growth; administration of propylthiouracil retards it. The effects seem to be more marked in the dorsal and head regions than in the flank or belly. Propylthiouracil does not prevent the activation of the follicle induced by plucking of the club hair. It is suggested that the hormonal effects may be produced indirectly through the dermis. Hypophysectomy increased definitive hair length in females but not in males; thyroxine also produced a small increase and propylthiouracil a decrease which proved to be significant in females, but not in males.