An experimental model using asplenic rabbits injected with radioactive amino acids has been developed to examine the density distribution of newly released platelets. Platelets from asplenic rabbits had a larger mean volume and greater protein content than those from asplenic animals. Radiolabel, indicative of new platelets, is preferentially incorporated into the most dense platelets during the early postinjection period. Platelets of intermediate density also demonstrated augmented early label incorporation compared to the lightest cells, In contrast, during the late postinjection phase, there is preferential labeling of the least dense platelets. The specific radioactivity of heavy and light platelets rises to approximately equal levels, and falls abruptly, but the peak activities occur at distinctly separate times consistent with the concept of platelet density modification during platelet aging. Although platelet biologic properties are broadly distributed, these studies support the concept that on the average young platelets have an increased density and become less dense as they circulate.