Studies were made of the early and delayed effects of <tex-math>${\rm H}^{3}{\rm TDR}\text{-labeled}$</tex-math> DNA on the replication kinetics of the ileal cells of 3-week-old male Wistar rats. Following 1 μCi/gm, there were three evenly spaced cycles involving the same cohort of cells. After 2 and 10 μCi/gm, cell replication occurred, but the regular polycyclical pattern observed after 1 μCi/gm was disrupted. The effects were greater with 10 μCi than with 2 μCi. The alterations in cycle pattern were due to radiation and not to thymidine pool dilution. Five weeks after 2 and 10 μCi/gm, when essentially all the previously labeled cells were lost, a labeling dose of 50 μCi of H3 TDR per rat was administered. At this time, the replication patterns were similar to those observed in 3-week-old rats following 1 μCi/gm. On the other hand, when 2 μCi/gm was given to 1-day-old rats, the studies made 3 weeks later, after a second label of 1 μCi/gm, revealed a persistently disrupted replication pattern. Little labeling of ileal cells remained 3 weeks after the initial 2 μCi/gm. There was no evidence of ileal cell necrosis in any animals. The results in ileal cells are considered in relation to previously published studies on coexistent hepatocytes.

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