A study was made of changes in blood fibrinolytic activity in a group of young male university students following varying levels of exercise. The subjects were divided into two groups. Those partaking in regular athletic programs were classified as athletes and those not as nonathletes. There was an increased fibrinolytic activity with increased severity of exercise in all subjects. Several of the nonathletic group showed a much greater maximum effect than their athletic counterparts, and all nonathletes showed the effect at much lower levels of exercise. Increase in fibrinolytic activity correlated well with increase in pulse rate. When euglobulin fractions from the plasma samples were subjected to microzone electrophoresis, there was a band change in the region of the beta globulins in many of the samples following exercise. This band change is apparent only when fibrinogen is present in the euglobulins and appears to be due to fibrinogenolysis as a result of the activation of the fibrinolytic system. These band changes were observed when the specific activity was as little as 0.058 unit of plasmin per milligram protein. Our results show that electrophoretic pattern changes of euglobulin fractions is a relatively sensitive and rapid method for detecting activation of the fibrinolytic system.