Thymic lymphopoiesis in 2 day old Swiss albino mice (Hale-Stoner strain) was studied autoradiographically based on the variation of mitotic labeling indices and intensities after a single intraabdominal injection of 3H-thymidine. Best estimates for the duration of various phases of the generative cell cycle are as follows: G2 for the majority of cells: 86-94 minutes, shortest possible duration for a small fraction of cells: 35-48 minutes; mitosis: 27-44 minutes; DNA synthesis: 7 hours; generation time: 9¼ hours. G1 could not be determined directly by the methods used but probably is short. The present data do not indicate different generation times for proliferating thymic lymphoid cells of unequal size although the very largest elements must be excluded from this statement because of rare occurrence. The results obtained in this study together with data on changes of thymic weight and proliferative activity in the neonatal period indicate that during the third day after birth, a fraction corresponding to one third of all lymphoid cells present in the organ at the end of this period is lost. In view of the extremely low pyknotic index in the thymus at this age, the present data provide support in favor of an extensive emigration of lymphoid cells being the major cause of this cell loss.