1. A single injection of orotic acid-6-C14 was administered to partially hepatectomized rats 24 hours after the operation. An average of 40 per cent of the dose of radioactivity was excreted in the urine during the first 4 hours, and an average of 20 per cent of the dose appeared in the respiratory CO2 within 20 hours.

  2. The liver contained 35–45 per cent of the dose of C14 30 minutes after the injection. Initially the labeled acid-soluble intermediates were incorporated into nuclear RNA at a rate greatly exceeding the appearance of C14 in the cytoplasmic RNA. Twelve hours after the injection, the specific activities of the nuclear and cytoplasmic RNA were nearly the same, and a parallel decrease in the specific activity of the two RNA fractions was observed thereafter. Sixty-four and 100 days after injection the RNA fractions contained very low amounts of C14.

  3. The isotope content of the DNA continued to increase during the first 24-hour period, and throughout the subsequent 100-day period the isotope level of the total liver DNA remained about the same.

  4. A time curve of the rate of DNA synthesis during the early period of liver regeneration has been described. No DNA synthesis occurred during the first 18-hour period postoperatively, a maximum rate of DNA synthesis was observed 24–30 hours after the operation, and incorporation of C14 into DNA continued extensively for the subsequent 2-day period.

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This work was supported by a grant (No. C-646) from the National Cancer Institute, National Institutes of Health, United States Public Health Service. Preliminary reports of this work were given previously (13, 14).

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