Blood coagulation, fibrinolysis, platelet count-aggregation and cortisol were assessed in 35 Finnish amateur runners aged 27 to 56 years (mean 40) and three aged 65, 67, and 82 who had run a non-competitive Marathon in 1975, 1976 and 1977 over the classical itinerary. After the run, in all 3 years, APTT showed shortening (p < 0.001); prothrombin time and plasma fibrinogen were not significantly altered; euglobulin lysis time was shorter (p < 0.001) and FDP increased (p < 0.001); PSPT became positive in all subjects, whereas the ehtanol gelation test remained negative; no cryofibrinogen was detected. Platelet count and aggregation showed increase (p < 0.001) in 1975 (extreme heat, 25 degrees C) but remained unaltered in 1976, 1977 (15-18 degrees C). Cortisol levels were always significantly increased -- more markedly in 1975. Women's responses were similar to those of men. A possible correlation between physical fitness and the responses of haemostatic balance is suggested and the influence of weather is discussed.