Platelet aggregability might be increased during physical exercise. This, in turn, has been explained by the elevation of plasma catecholamines and by the state of lactic acidosis, which occur at high exercise intensities. The purpose of this investigation was to study the relationship between changes in platelet aggregability and exercise intensity, the latter being determined in reference to the anaerobic threshold (AT). Each of sixteen male subjects performed an incremental exercise test in order to determine both (a) his running velocity (VAT) corresponding to his anaerobic threshold and (b) his running velocity (V4mM) corresponding to 95% of his running velocity eliciting a blood lactate concentration of 4 mM.l-1. Three and six days after this preliminary test, respectively, each subject performed an exercise test of 30 minutes, at a constant running velocity of either VAT or V4mM. Running velocity for each day's test was randomly assigned. Both capillary and venous blood samples were collected immediately before and immediately after each test, and after 30 minutes of recovery from each test, respectively. Capillary blood samples were obtained for determination of blood lactate concentration, whereas venous blood samples were obtained for determination of platelet count, and platelet aggregation in response to ADP and collagen, respectively. Platelet count significantly increased (p < 0.001) immediately after the 30-minute-tests at either VAT and V4mM, remaining elevated (p < 0.05) after 30 minutes of recovery from the tests at V4mM. The results did not evidence any significant increase in platelet aggregability with exercise, except for aggregation response to ADP immediately after the tests performed at V4mM (p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)