Diurnal variations in radiosensitivity of mouse intestine.

Crypts in the mouse small intestine show a diurnal rhythm in radiosensitivity and are most sensitive in the middle of the dark period, when mitotic activity in the crypt is high. Analysis of crypt survival on a cellular basis shows that the changes in cryptogenic cell radiosensitivity are dose-modifying between 1,000 and 1,450 rads gamma rays such that the DO value varies between 85 rads and 140 rads.