The alkaline RNAse level of mucosal scrapings from rat small intestine was increased after 700 R of whole-body X-irradiation, when expressed as total intestinal content or relative to DNA or protein levels; this increase occurred as early as 3 hours after irradiation. Increases also occurred after local irradiation of the head. Both the inhibition of neuroendocrine reactions in mature rats by the administration of drugs, and the use of infant rats in which these reactions do not occur, suppressed the alkaline RNAse increase after irradiation, thus indicating that neuroendocrine reactions may be the cause of the enzyme increase. The application of ACTH, hydrocortisone, or general stressing agents did not produce any increase in the enzyme level, but the radiomimetic chemicals Chlorambucil and Myleran did cause increases similar to those induced by radiation.

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