ALTHOUGH renewed efforts have been made towards an understanding of the function(s) of the thymus, the exact mechanisms involved remain unclear. It is not known whether the thymus serves primarily as a source of immunologically competent cells, or whether it produces a substance or substances and/or contact factors which may control proliferation and immunological maturation of peripheral lymphoid cells. Combinations of such suggestions have been made repeatedly1,2. A meaningful evaluation of the existence and magnitude of thymic cell migration during the perinatal period of life requires more information on at least the following thymic growth characteristics as a function of age before and after birth: (a) the rate of thymic growth; (b) proliferative activity of thyrnic lymphoid cells; (c) the generation time of thymic lymphoid cells; and (d) a quantitative estimate of intrathymic cell death. We report here an investigation of growth and proliferative activity of the thymus of mice ranging in age from 3 days before to 6 days after birth.