FUNCTIONAL activity of the hair follicle is known to be a cyclical phenomenon in many species1,2, involving periods of active growth and periods of apparent inactivity. The mechanism controlling this cycle is cutaneous, since hair-growth occurs in vitro3–5. The nature of this control mechanism is unknown. Chase1,6–8 has suggested the growth-cycle is caused by the rise and fall in the activity of an ‘inhibitor’. This proposal, however, contains certain limitations. Although Chase claims his hypothesis does not exclude the participation of a ‘stimulator’, its presence is an essential premise of his argument. The wane of ‘inhibitor’ activity would, in itself, be incapable of initiating a growth-cycle, for within the follicle there must exist a propensity to grow. Presumably this is manifested when the activity of the hypothetical ‘inhibitor’ declines to a threshold-level. This growth propensity necessarily requires a cause, in effect the action of a ‘stimulator’.