The pathogenesis of atherosclerosis, a major cause of age-related mortality, remains poorly understood. Although platelets and their products, including thromboxane A2, may be of importance in this process, little is known about eicosenoid biosynthesis and platelet function with increasing age. In order to address the hypothesis that platelet activation increases with age, we measured various indices of platelet function in a group of apparently healthy individuals over the age of 50 years. The circulating platelet aggregate ratio, plasma beta-thromboglobulin and threshold aggregating concentration of arachidonic acid were similar to those in healthy subjects aged less than 40 years. Although the bleeding time (168 +/- 24 vs 300 +/- 24 seconds) was significantly (p less than 0.001) shorter in the older volunteers this may be unrelated to platelet function and merely reflect age related changes in skin and/or vascular function. To further assess platelet and vascular function in vivo, we measured excretion of the major thromboxane and prostacyclin metabolites in urine, 2,3-donor-thromboxane B2 (Tx-M) and 2,3-dinor-6-keto-PGF1 alpha (PGI-M). Both Tx-M (223 +/- 22 vs 152 +/- 19 pg/mg creatinine; p less than 0.005) and PGI-M (198 +/- 21 vs 121 +/- 13 pg/mg creatinine; p less than 0.005) excretion were significantly higher in the older volunteers. These subtle but significant changes in eicosenoid biosynthesis are consistent with the presence of platelet activation in vivo increasing with age in apparently healthy individuals.