The diurnal increase of fibrinolytic activity in the forenoon was investigated after alpha- and beta-adrenergic blockade in 15 volunteers. Four milligrams propranolol was injected i. v. alone or in combination with 20 mg phentolamine. Neither the beta-blocking agent, propranolol, nor the combination with the alpha-blocker, phentolamine, had any significant effect on the diurnal increase of fibrinolytic activity.