Natural inhibitors of endogenous fibrinolysis may displace the hemostatic equilibrium toward thrombosis and favor events such as acute myocardial infarction, sudden cardiac death and stroke, where a thrombotic process is known to occur.^1,2 The clinical incidence of these syndromes shows a circadian distribution with highest frequency in the morning.^3,5 These observations prompted us to investigate possible circadian changes of blood fibrinolytic activity in normal subjects. Two major components of the fibrinolytic system, tissue-type plasminogen activator (t-PA) and its fast-acting inhibitor (PAI), were measured with specific assays. This study reports markedly reduced fibrinolytic activity during the early morning hours related to increased plasminogen activator inhibition.