More than 1000 Chinese hamsters, 80 days old, were used to examine the radiation response of Cricetulus griseus at different times during a 24 hour day. The <tex-math>${\rm LD}_{50/30}$</tex-math> values were calculated to be 954 rads (1 AM), 889 rads (9 AM), 823 rads (11 AM), 886 rads (1 PM), and 882 rads (3 PM). Three studies were made to attempt to explain the differences in radiosensitivity: (1) examination of the mitotic indices of white blood cells (WBC) in the bone marrow which showed that the rate of mitosis fluctuated markedly over a 24 hour period, the highest (11 AM) and lowest (1 AM) mitotic index corresponding to the lowest and highest <tex-math>${\rm LD}_{50/30}$</tex-math> value, respectively, revealing a linear correlation between <tex-math>${\rm LD}_{50/30}$</tex-math> values and rates of mitosis; (2) analysis of the peripheral blood of hamsters irradiated at a time when the mitotic index of WBC was either low or high which showed that the restoration performance of the circulating WBC density coincided exactly with the inverse of the rate of mitosis during exposure; and (3) calculation of RBC production after exposure which showed that the rate of59 Fe uptake is higher in animals irradiated at 1 AM than 1 PM. This study clearly points out the importance of exposing the animals at precisely the same time each day if more than one exposure is required.

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