With a view to investigating whether in circadian variations of platelet aggregation (PA) and fibrinolytic activity there is an elevated risk period for incidence and development of ischemic stroke, 25 healthy subjects (5 females and 20 males), their age ranging from 29 to 51, were exposed to the analysis of PA, euglobulin lysis time (ELT), fibrinogen degradation products (FDP), antithrombin III (AT III) and heparin tolerance test (HTT) in blood samples drawn by venepuncture at 08.00, 11.00, 13.00, 15.00, 17.00 and 18.00 h; beside these intervals in the case of 10 healthy males, whose age ranged from 32 to 45, blood samples were taken at midnight as well. The group of 25 subjects comprised those who usually worked daily and nightly shifts, as well as those who were either at bed rest or doing their duties during daytime. The findings of this investigation have demonstrated that all the parameters studied exhibited circadian variations irrespective of sex, age or daily/nightly activities of the subjects. The most pronounced PA interval, which was not accompanied by corresponding increase of fibrinolytic activity, was that around 11.00 h and it is marked as the highest risk period for onset of ischemic stroke.