Abstract
1. Comparisons were made between the livers of pregnant and non-pregnant rats, all of which were fed on a ration of 20 g food per day.2. In the second half of pregnancy there were marked increases in the weight of the liver and in its total content of protein, RNA and DNA. RNA concentration increased from the 15th day of gestation.3. Between the 12th and 18th day of pregnancy liver weight, total RNA, totla DNA and mean liver cell nuclear volume increased in parallel at approximately the same rate of 6-10% per day.4. Ornithine decarboxylase activity was enhanced in pregnancy by the 5th day of gestation and rose to peak activity at the 18th day.5. The mitotic index of hepatic parenchymal cells was elevated in the first and last stages of pregnancy. The later peak in mitosis was associated with a rapid fall in mean nuclear volume.6. There was histological evidence of cell turnover in the liver of pregnant rats; enlargement of the liver was associated with both hyperplasia and cell deletion.