Abstract The DNA contents of individual nuclei of rabbit mammary gland epithelial cells at various physiological stages of development were measured with a microspectrophotometer. The results showed the Feulgen nucleal reaction to be a reliable method for quantitative work and reproducible when carried out under standardized and rigidly controlled conditions. The DNA contents of nuclei of the epithelial cells are not equal. Significant differences in DNA content per nucleus were present among individual animals, among glands of the same animal, and among areas within the glands. The proliferative activity of the cells was studied by the administration of tritiated thymidine to the rabbits 1 h before they were sacrificed. Radioautography of the mammary gland tissue was carried out to detect incorporation of thymidine into nuclei that synthesize DNA. It was found that a considerable proliferative activity takes place in the adult virgin rabbit. The largest proliferative activity occurs during the first half of pregnancy. Proliferative activity exists during the second half of pregnancy, but at the slower rate. The activity increases markedly one day prior to parturition, though it is somewhat lower than that during the first part of pregnancy. The high rate of proliferative activity continues during the day of parturition. A sharp drop occurs on the first day of lactation and remains low during lactation. There appeared to be a relationship between the DNA content per nucleus and the secretory activity of the cell. The content was high at times of high secretory activity and low at times of secretory quiescence. An inverse relationship between the proliferative activity and the quantity of DNA per nucleus also existed. The presence of non-genetic DNA in excess to the genetic diploid quantity is suggested. This additional DNA is postulated to be responsible for the increased DNA content per nucleus and may play a role in the secretion process.