The application of H3-thymidine results in labeling of those nuclei of cells in which deoxyribonucleic acid (DNA) is synthesized during the interval between application and the sacrifice of the animal (1-3). This paper reports autoradiographic investigation with H3-thymidine of rats and mice. This method permits a more exact statement of the number of dividing cells than does the microscopic estimate of mitosis. The latter method is practically impossible in tissues with small fusiform cells. Moreover, it is possible to obtain information about the relative time of DNA synthesis in different cells.