The influence of acetylsalicylic acid (ASA) on alterations of the platelets resulting from physical exertion was studied in 10 healthy volunteers. Before and 2 h after the administration of placebo or 1 g ASA, venous blood samples were taken for platelet counts, thrombelastography (TEG) and tests of ADP- and epinephrine-induced platelet aggregation. The blood in which the 2-h values were determined was sampled at the highest work-load during sub-maximum exercise on a bicycle ergometer (5 min each at 50, 100 and 150 watts). ASA had no effect on the alterations of the platelets due to physical effort: the increase in the first phase of epinephrine-induced aggregation, the elevation of the platelet count and the hypercoagulability demonstrable by TEG still persisted after the administration of ASA. Some effects were noted, however, that are also observed after the administration of ASA at rest: the second phase of epinephrine-induced aggregation was suppressed and disaggregation after induction with ADP was accelerated.