The increase in the number of nuclei with age was estimated from the deoxyribonucleic acid (DNA) content of or-gans and tissues of young, growing rats and expressed as the daily nuclear addi. tion rate. A search was also made for dividing cells, especially in radioauto-graphs after H®-thymidine injection and after administration of colchicine. The use of this drug made it possible to measure the daily mitotic rate. Pre-liminary results had suggested that the cell populations of the embryo increase exponentially, but, toward the time of birth, exponential growth ceases. The cell populations were then investigated with the use of rats ranging in age from about 7 to 90 days. In some popula-tions, no mitotic activity was detected and the DNA content did not change. Hence, the number of cells remained constant during the period investigated. These cell populations are referred to as static, e. g., neurons. In other organs and tissues, the DNA con-tent and, therefore, the number of cells kept increasing, though at a slower and slower rate with age. This increase could occur in two ways. In the first case, mitoses were scattered and their number accounted for the increase in DNA content (and, therefore, for the addition of nuclei). Hence, all cells added by mitosis were retained. These cell populations are referred to as expanding, eg., parenchymal cells of liver and kidney, muscle fibers, etc. In the other case, mitoses were abundant and their number greatly exceeded that required for the increase in DNA content (and, therefore, many more cells were produced than were retained in situ). Cell losses must have taken place, in such a manner that the high cell production only slightly exceeded the losses. These cell populations are referred to as renewing, eg, cells of epidermis, intestinal epithelium, thy-mus, etc. In neoplastic cell populations, proliferation of the cells and their progeny tended to continue indef. initely. Even, if some cells stopped di-viding and some degree of renewal occurred, cell production greatly ex-ceeded the cell losses.--Nat Cancer Inst Monogr 14: 119–150, 1964.