The effect of the number of micromoles of thymidine on the metabolism of tracer quantities of <tex-math>${}^{3}{\rm HTdR}$</tex-math> were studied in mice after oral administration of 1.0 μCi of tritium incorporated into different amounts of thymidine. The quantity of nonvolatile radioactivity in tissues versus time suggested the presence of tritium in two very different metabolic components. Early labeling of tissue was in proportion to the amount of thymidine administered. Time studies revealed that increases in the total quantity of thymidine (lower specific activity <tex-math>${}^{3}{\rm HTdR}$</tex-math>) administered led to a higher fraction of radioactive tracer label in the more stable component of tissue activity. Radiation effects were detected by comparing the retention of label following administration of 1.0 and 8.3 μCi of <tex-math>${}^{3}{\rm HTdR}$</tex-math> in the same number of micromoles of thymidine. When 8.3 μCi was administered, a rather rapid rate of decline of activity was observed in the component of tissue activity in which the tracer label is usually very stable. Thus, a radiation effect is present when <tex-math>${}^{3}{\rm HTdR}$</tex-math> is administered to mice in an amount as low as 0.3 μCi per gram body weight.

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