The early kinetics of mitogen-stimulated spleen cells from young adult and aged NMRI mice were investigated. Using RNA synthesis as measured by cytofluorometry and 3H-leucine and 3H-thymidine incorporation as parameters, it was apparent that age-related changes occur at different levels during mitogen stimulation. First, in aged mice fewer cells per 106 spleen cells are being activated by mitogen, and, second, fewer of the activated lymphocytes will proliferate. However, the cells which initiate synthesis of RNA, protein, and DNA apparently behave in identical manners, chronologically, as cells from young adult mice.