The thyroid 131I consumption, the hypophyseal thyrotropic hormone content and the blood serum total thyroxin and triiodothyronine concentrations were studied in equal groups of healthy humans and donors, living in two cities with an enhanced or decreased fluorine content in drinking water. Iodine deficiency and adaptive amplification of the hypophyseal-thyroid system, not ensuring an absolute compensation, were found in the citizens, using drinking water with an increased fluorine content, accompanied by an augmented incidence of functional disturbance, which structure remained relatively unchanged.