A subhuman primate model was developed to ascertain whether or not platelet heterogeneity could be explained by aging in the peripheral circulation. Density-dependent platelet cohorts, postulated to represent cells of different ages, were isolated on isosmolar arabinogalactan gradients and labeled with radiochromium. Mean platelet lifespan was measured for the different density cohorts, and simultaneous sequential density distribution analysis was performed to follow changes in cell density during aging. The average mean lifespan of light platelets was 74.6 hr, compared to 313.6 hr for heavy platelets. After injection, labeled light platelets were recovered only in the gradient light region, in contrast to labeled heavy platelets, which were initially restricted to the dense region and progressively migrated to the light region during their lifespan. This study supports the hypothesis that platelet age in unstressed primates correlates with cell density and provides a rationale for the use of “age-dependent” markers to estimate platelet turnover rates.