The effects of intravenous infusions of adrenaline alone, and after alprenolol (Aptin^®) pre-treatment, on whole blood clotting time, factor VIII, fibrinolysis, partial thromboplastin time (PTT), platelet count and fibrinogen have been studied in four healthy, fasting male subjects by double-blind technique using saline infusions as control. The adrenaline infusion caused increases in factor VIII levels, fibrinolysis and platelet counts and a decrease in whole blood clotting time. Alprenolol inhibited the effects of adrenaline on factor VIII and fibrinolysis in all subjects, and the effects on whole blood clotting time in all but one subject. The adrenaline-induced rise in platelet count was not significantly influenced by alprenolol pre-treatment. No effects were seen of the alprenolol infusion itself.