To understand the effects of ionizing radiation on thyroid cells, we investigated the role of p53 in mediating apoptosis and in DNA repair following in vivo and in vitro irradiation of thyroid cells. In vitro exposure of human thyroid cells to ionizing radiation of up to 5-8 Gy failed to induce apoptosis in primary cells. The same results were obtained when the thyroid gland was irradiated in the intact rat. To explore the mechanism of failure of the wild-type p53 in inducing apoptosis in thyroid cells, we investigated the expression of apoptosis-related genes, bax, bcl-2 and fas/APO-1 following irradiation or induction of temperature-sensitive p53. The expression of Bax, Bcl-2 and Fas/APO-1 in human primary cultured thyroid cells did not change after irradiation. To further confirm the results, we established a clonal cell line (tsFRO) in which a temperature sensitive p53 (Val138) expression vector was stably transfected to a thyroid carcinoma cell line lacking endogenous p53. Incubation of tsFRO cells at the permissive temperature for three days, however, did not induce apoptosis although G1 arrest was noted. Although enhanced expression of the bax mRNA level was observed, the expression of Bax, Bcl-2 and Fas/APO-1 protein did not change by shifting tsFRO cells to permissive temperature as well as irradiated primary cells. Furthermore, DNA end-jointing ability was examined by transfection of linearized luciferase plasmid into tsFRO cells. Increased luciferase activity occurred when the cells were cultured at the permissive temperature, indicating that the wild-type p53 enhances DNA end-jointing activity. Our results indicate that the wild-type p53 does not lead to apoptosis but facilitates DNA end-jointing in thyroid cells. These results may reflect specific responses in thyroid cells following irradiation.