SummaryWe studied the effect of acute exercise on the ability of thrombin to activate plasma factor VIII (FVIII) activity in 20 healthy males. The subject showed an average exercise-related increase in FVIII activity of 54.5±8.2% over pre-exercise FVIII activity (p<0.001). When exposed to the same concentration of thrombin, post-exercise FVIII activity showed greater enhancement than pre-exercise FVIII activity: 157.1±12.8% increase in activity versus 117.3±9.9%, respectively (p<0.01). The degree of the potentiated thrombin effect in post-exercise samples relative to pre-exercise samples was linearly correlated with the degree of the exercise-related increase in FVIII activity. Taken together with our previous observations that the extent of thrombin enhancement of FVIII activity varies inversely with the mole ratio of FVIII/von Willebrand factor subunits to thrombin, these findings imply that release of FVIII does not occur during exercise, and that the exercise-related increase in FVIII activity results primarily, if not completely, from activation of already circulating but inactive FVIII.