Acute 12-minute exposure of laboratory rats to microwaves with specific dose rate (SAR) of 30 W/kg that exceeded a basal metabolism caused a transient response of hyppophysis-thyroid system of compensatory-adaptive character. Prolonged exposure to microwaves of less intensity (SAR = 6 W/kg, which approximately corresponds to basic metabolic rate for these animals) caused insufficiency of the function of thyroid control in a form of primary hypothyroidis.