Thrombopoiesis after a single dose of 2,4,6,8 and 10 Gy gamma-ray total body irradiation was studied by means of ,35,S incorporation technique in mice. It was found that the post irradiation change of thrombopoietic activity ran a polyphasic course. The thrombopoiesis was stimulated 2 days after irradiation except in the 10 Gy group. The time of onset of declination, the nadir and the speed of recovery of thrombopoiesis were correlated with the dose of irradiation. Meanwhile, the slope of declination was independent of the dose, and the nadir was reached arround the 8 th post irradiation day in all dose groups. Thrombopoiesis could recover to normal level spontaneously within the dose range up to 8 Gy. An overstimulation of thrombopoiesis was seen in the recovery phase after 2-6 Gy irradiation, followed by a depressive phase. The peripheral platelet count followed a similar course as the thrombopoietic activity, with a smaller range of variation. The relationship between thrombopoiesis and peripheral platelet count and the characteristics of the change of thrombopoiesis following different doses of irradiation are discussed.