SummaryThe rate of enzymatic incorporation of 3H-labelled thymidine into primer DNA was studied after the DNA had been irradiated with doses up to 360 000 roentgens of gamma-rays. Non-irradiated DNA-synthesizing enzyme obtained from regenerating rat-liver was used together with DNA prepared by the method of Kay, Simmons and Dounce (1952) in the incubation system described by Bollum (1959). An initial increase followed by a decrease was noted in one sample of DNA, whereas a second less-highly-polymerized sample showed only a decrease in the rate of incorporation. Isolated calf-thymus nuclei showed no changes in the rate of incorporation of 3H-labelled thymidine or cytidylic acid after irradiation with very high doses of gamma-rays. The results of these in vitro studies do not provide support for the concept that damage to DNA preventing replication is a factor in the decrease in DNA-synthesis noted in irradiated cells.