Alterations of platelet function and blood coagulation may occur with exercise or β-adrenoceptor blockade. To determine if β-blockade could modify exercise-induced changes in haemostatic factors we performed a double-blind study of actue strenuous exercise in normal males with and without β-blockade. Exercise increased prostacyclin and plasminogen activator levels but there was no evidence of thrombin generation as indicated by unchanged platelet aggregation responses, β-thromboglobulin and íibrinopeptide A levels. The only alteration in coagulation by β-blockade was a reduction in the factor VIII:C and VIII:RAg rise after exercise and this modification may be relevant to the protective effect of these drugs in patients with coronary artery di