To resolve the details of recovery processes from radiation injury after whole-body irradiation in the fish Oryzias latipes, 3 or 4 kR of x-rays was given in two divided doses. The interval between the two fractions varied from 5 minutes to 30 days, and the water temperature during the intervals was adjusted to 15° or 24°C. After the second irradiation, the 30-day mortalities in different groups of fish were compared. On the basis of the findings from these experiments and histological studies on the intestine and hematopoietic organs of irradiated fish, three phases in the recovery processes were formulated. During the first 3 hours after irradiation, a rapid recovery from the injury took place, reflecting intracellular repair in tissue involved. This was followed by the second phase, showing characteristic fluctuations in magnitude of injury. The time required for the peak of recovery and the following depression to occur was longer at 15°C than at 24°C. It seems probable that during this phase cells appeared to be synchronized with respect to cell division and the cells might exhibit difference in radiosensitivity according to the stage of cell cycle. The third phase began more than 3 days after irradiation. Thus, the recovery from radiation injury appears to be related to repopulation of cells in critical organs, and at least three different phases are involved in the events.

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