The aim of this study was to evaluate diurnal variations in the haemostatic response to submaximal

exercise performed by young, sedentary men. Fifteen healthy young sedentary males aged 25.6 ± 1.34 (mean

± SD) years performed two exercise sessions, morning and evening, at 70% of maximal oxygen consumption

(VO2max) on a cycle ergometer for 30 min. Platelet count (PC), activated partial thromboplastin time (aPTT),

prothrombin time (PT), fibrinogen, tissue plasminogen activator (tPA) and plasminogen activator inhibitor-1

(PAI-1) activity were measured as dependent variables. Exercise produced significant increases in PC and fibrinogen

for both sessions (P≤0.05), which returned to the resting values after recovery only in the evening session. APTT

and PT shortened immediately after exercise, which remained after recovery for both sessions (P≤0.01). Exercise

presented significant increases in tPA activity (P≤0.001), which returned to the baseline after recovery in both

exercises. PAI-1 activity was significantly higher during the morning than evening (P≤0.05), but no longer

demonstrated exercise-related changes. It was found that exercise caused activation of both coagulation and

fibrinolysis processes, partly related to the time of the day when the exercise was performed.