Population of the thyroid C-cells, normal and at sympathectomy has been analyzed in 75 white male rats at the age of 1, 3 and 6 months by means of electron microscopical, morphometrical and radioimmunological methods. Partial sympathectomy has been performed using subcutaneous injection of guanethidine (15 mg/kg of body mass) during 14 days after birth. During the period from 1 up to 6 months of life in intact rats a decrease in C-cells functional activity is observed. Under conditions of sympathectomy in 30-day-old animals decreasing extrusion processes of the secretory material are observed. During successive periods of life (3 and 6 months) mechanisms of paracrinic evacuation of hormonal products enhance considerably, nuclear volume of the cells and number of C-cells in the field of vision increase. Their hyperplastic alterations in the sympathectomized thyroid gland are more pronounced in 3-month-old animals.