In five volunteers fibrinolytic activity has been measured five times over a period of six weeks during increasing values of circadian rhythm. Euglobulinlysis-time, paper fibrinolysis, fibrinogen, plasminogen, alpha1-antitrypsin, alpha2-makroglobulin, thrombino-coagulase-time and fibrin-split-products were used in one study. An increase of fibrinolytic activity in the mooning was observed in all volunteers. Intraindividual variation of values is considerable but less than variation of values compared interindividually. Moreover variation was smaller at noon than in the time before noon. In clinical trial of the short-time effect of drugs the period of the plateau of the fibrinolytic activity should be used because of its smaller variation of values. Furthermore, in this period the intraindividual variation of the fibrinolytic activity is smaller than during increasing slope of the circadian rhythm of fibrinolysis. Intraindividual control should be granted.