The quantitative changes in nucleic acids, 3H-thymidine incorporation and the colony-forming capacity of haemopoietic stem cells were followed in the blood of rats during 28 days after a single exposure to 5.5 Gy of X-radiation. The concentration of nucleic acids, especially that of DNA, decreased sharply already during the first day, falling to the minimum values by day 10 after irradiation. The RNA concentration recovered by day 21 and the DNA concentration by day 28. 3H-thymidine incorporation into DNA in the blood of the irradiated animals was increased, especially at 10-14 days. From the increased incorporation of the precursor into DNA after short-term (4h) labelling in vitro and without additional stimulation we judged that at that time the DNA-synthesizing cells occurred in increased numbers in the blood of the irradiated rats. Simultaneously we examined the colony-forming capacity of the blood (CFU-S) with an attempt to contribute to the elucidation of the function of DNA-synthesizing cells. The changes in colony-forming capacity proceeded in parallel to changes in 3H-thymidine incorporation, with a maximum number of CFU-S by day 14 after irradiation. The results suggest that DNA-synthesizing cells in the blood of the irradiated rats represent circulating haemopoietic stem CFU.