New Zealand rabbits, fasted for 12 hours, were subjected to 500 rads of whole-body irradiation.${\rm K}^{+}\text{-palmitate-}1\text{-}{}^{14}{\rm C}$ oxidation was assayed with the 600 × g supernatant of thymus and liver homogenates, in the presence of ATP, at various time intervals from irradiation. For a period of 24 hours following irradiation, oxidation by liver preparations was not significantly affected. The rate of oxidation by thymus was decreased to less than one-third of the control value within 12 hours from irradiation and, at 24 hours, was almost completely abolished. Increased ATP concentration could increase only to a small extent the oxidation by thymus preparations of irradiated animals. Oxidation by isolated thymus mitochondria was also inhibited by irradiation. Counting of the water-soluble oxidation products of$\text{palmitate-}1\text{-}{}^{14}{\rm C}$ suggests that the inhibition is not due to the impairment of the reactions of the citric acid cycle. The non-esterified fatty acid concentration of thymus was not altered at 12 hours following irradiation. Esterification of${\rm K}^{+}\text{-palmitate-}1\text{-}{}^{14}{\rm C}$ into the thymus lipids was not affected 12 hours after irradiation.

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