The morphological changes and proliferative activity of FRTL-5 cells irradiated in vitro with one dose of 0, 8, 12, 16, 24 or 32 Gy were studied morphometrically 7, 14 and 21 days after irradiation. The number of cells irradiated with 24 Gy and 32 Gy decreased during the 3 weeks of the experiment. Morphometrical analysis confirmed that the cells repaired their injuries up to 16 Gy of irradiation, while doses of 24 and 32 Gy damaged them irreversibly. Thyroglobulin was detected in cells after all doses of irradiation.