Acute gamma irradiation affected the incorporation of the thymidine analog <tex-math>$5\text{-}{}^{131}{\rm iodo}\text{-}2^{\prime}\text{-deoxyuridine}$</tex-math> (<tex-math>${}^{131}{\rm IUDR}$</tex-math>) by cotton rats, Sigmodon hispidus. Absorbed doses of 100, 500, and 1000 rads administered 24 hours before injection depressed whole-body measurements of radioactivity retained at 48 hours postinjection to 76%, 38%, and 28%, respectively, of control values. The greatest depression of total activity retained within tissues occurred in the intestines, where activity levels were reduced to 47%, 18%, and 1.5% of control values. Since the data included responses from maximum incorporation in controls to minimum incorporation in animals irradiated at 1000 rads, a regression of activity remaining in the intestines as a function of total body burden was calculated to interpolate responses at intermediate dose levels. The in vivo measurements were a useful index to radiation insult at doses below 1000 rads. The method described should serve as a valuable adjunct to other techniques for measuring the effects of ionizing radiation in free-ranging mammals at dose levels that are too low to induce gross pathological responses or death. At the highest dose, activity accumulated in the excretory pathways and the gastrointestinal contents; the latter may have been due to a delayed emptying syndrome of the tract.

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