The increased irregularity of rat thymocyte nuclei observed by electron microscopy after whole-body x-irradiation with 1000 R was assessed quantitatively by the use of the I value, the ratio of the perimeter to the area of the nucleus. The mean I values for both cortical and medullary thymocytes registered an increase 1 hr after irradiation. The value for medullary thymocytes decreased at 3 hr, while it remained unchanged for cortical thymocytes. Furthermore, a decrease in number of normal cortical and medullary thymocytes as a function of time after irradiation with 1000 R was observed by electron microscopy. Analysis of the relationship between the increased nuclear irregularity and pyknosis led us to confirm quantitatively the previous findings of the present authors that this increase in irregularity is the process preceding pyknosis.

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