Circadian stage dependence in radiation: response of dividing cells in vivo.

1. When the mouse corneal epithelium wa irradiated with one dose or ionizing radiation, the effect on mitosis varied depending upon the time in the 24-h period when it was irradiated. 2) Release of the radiation-induced mitotic block, as measured by appearance of a recovery wave of mitotic cells, occurred only during the daily increase in mitotic index in the control animals. 3) The entire wave of recovery occurred within the same time as the controls, and no rebound was seen. 4) Mitotic delay was dose-dependent. 5) The results of these experiments emphasize the importance of considering the circadian system when studying cell division in vivo.