To measure the changes in DNA binding activity of CREB and the ratio of cAMP/cGMP in thymocytes after whole body irradiation (WBI) with X rays, as well as to discuss the relationship of the activation of CREB with cAMP signal pathway. Changes in DNA binding activity of CREB and the concentration of cAMP and cGMP in mouse thymocytes were respectively assayed by electrophoresis mobility shift assay (EMSA) and competitive protein binding assay. The increased activity of CREB DNA—binding was upregulated after irradiation with larger doses of X rays (0.5-6.0 Gy) , with the upregulation being most significant after irradiation with 2.0 Gy X rays. Following WBI with 0.075Gy, there was only a brief decrease followed by slight increase in the DNA binding activity of CREB, while the decrease in cAMP/cGMP ratio reached the lowest value at 24h. These results suggested that the slight increase in the DNA binding activity of CREB observed after irradiation with 0.075 Gy may not involve the cAMP signal pathway, while the latter was probably involved in the marked increase in the DNA binding activity of CREB induced by WBI with higher doses of X rays.