BACKGROUND This study examined the effect of age and aerobic exercise performed on a regular basis on human platelet protein kinase C (PKC) activity and translocation. METHODS Blood platelets were obtained from young (20-36 years) and older (61-78 years) healthy male human subjects. Platelets were incubated with PMA, 5-HT, or thrombin; PKC activities were measured in partially purified extracts of cytosolic and membranous fractions. RESULTS Platelet PKC activities associated with both the membranes and the cytosol cellular fractions were significantly reduced in older nonexercisers. Redistribution of platelet PKC activity elicited by stimulating the cell surface receptors for 5-HT or thrombin or by direct PKC stimulation (PMA) was found to be reduced in the elderly subjects. Maintenance of aerobic fitness in the older group of exercisers partially prevented the age-associated decline in platelet PKC activity and in stimuli-induced enzyme redistribution. CONCLUSIONS These results indicate that platelet PKC activity and its translocation may be biological markers of aging and that aerobic exercise may serve to slow the rate at which enzyme activation declines during senescence.