The rate of 3 H-thymidine incorporation into spleen and thymus DNA of whole-body-irradiated DBA/2 mice has been investigated. We observed a diphasic incorporation pattern in both the DNA fraction that was insoluble in 0.14 M NaCl and the fraction that was soluble in 0.14 M NaCl and insoluble in 0.2 N PCA (free DNA). An initial decrease was observed when 3 H-thymidine was injected during the first 15 minutes after irradiation. The decrease of incorporation was followed by an increase when the thymidine was given between 15 and 30 minutes after irradiation, and after this time a continuous decrease in incorporation was observed. It was also found that after irradiation the newly synthesized DNA was more readily extracted from the nucleoprotein complex by sodium-trichloroacetate-phenol than was preformed DNA. These effects were found to occur prior to a gross change in the lability of the preexisting DNA-protein complex.