Neurons of the mouse were labeled with [ 3 H]thymidine during their prenatal period of proliferation. The 3 H activity of the Purkinje cell nuclei was then studied autoradiographically 8, 25, 55, and 90 days after birth. The measured grain number per nucleus decreased by about 14% between the 8th and 25th postnatal days and then remained constant up to 90 days. There was no significant decrease of the 3 H activity of the Purkinje cell nuclei after correction of the measured grain number per nucleus for increasing nuclear volume of the growing Purkinje cells and for the influence of [ 3 H]ß self-absorption in the material of the sections. Injection of a high dose of [ 3 H]thymidine into young adult mice did not result in 3 H labeling of either Purkinje or other neurons in other brain regions. The results agree with the concept of metabolic stability of nuclear DNA. "Metabolic" DNA could not be observed in these experiments.