Irradiated rats (200 to 600 R) have an abnormal pattern of appearance of14 CO2 in the breath following intravenous administration of L-histidine ($\text{imidazole-}2\text{-}{}^{14}{\rm C}$) which is characterized by a prolongation of the time at which the maximum rate of14 CO2 excretion occurs and a diminution in the total fraction of the administered14 C appearing in the breath during the initial 80 minutes of the study. A possible explanation for these results is suggested to be the radiation inactivation of tetrahydrofolic acid or the processes responsible for its production.

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