Rats were treated with cortisone, x-radiation, and both agents in combination, and the effect noted on the DNA content of hepatocytes. Nuclei were enumerated both in whole liver homogenates and following isolation. The incorporation of P32 into DNA was also studied in relation to these agents. The following observations were made:—

1.The DNA content of nuclei fell both during cortisone administration and following x-radiation. In the former instance, the fall was progressive with continuing administration of hormone; in the latter instance, there was a return to normal 5 days after radiation.

2. Cortisone administration to x-radiated rats caused a fall in DNA/nucleus and prevented the return to normal at 5 days.

3. There was no evidence that the effects of cortisone and x-rays were additive in reducing DNA/nucleus.

4. These data indicate an alteration in DNA/nucleus, but simple changes in ploidy cannot be excluded. Either explanation requires that the agents used affect the DNA of non-regenerating nuclei.

5. Cortisone interfered with the incorporation of P32 into the DNA of regenerating liver. Only a small effect on DNA synthesis in resting liver was observed with cortisone or x-radiation.

6. DNA content of nuclei returned to normal 5 days after x-radiation and 3 days after discontinuance of cortisone. Slight increase in the incorporation of P32 by DNA was observed during recovery phases.

7. The hypothesis is proposed that the apparent losses and increases in DNA/nucleus were due to depolymerization and repolymerization of DNA. Following x-radiation and/or cortisone administration, it is proposed that some DNA is depolymerized and becomes cold acid-soluble and dissociated from organized chromatin. Later, conditions are such that this degraded DNA is repolymerized.

8. These data might be interpreted to indicate that a portion of the DNA is not essential to cell integrity; alternatively, there may be two or more species of DNA, one of which is more readily affected by the agents investigated in the present report.

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