The effects of aerobic and anaerobic physical conditioning on fibrinolysis were studied before and immediately after physical exercise. Moderately active controls (group A) were compared with aerobically- (group B) or anaerobically-conditioned (group C) subjects. Comparison of the resting parameters revealed that FgDP were significantly higher in group B as a compared to groups A and C. FbDP did not significantly differ between groups B and C and were significantly lower in group A. t-PA antigen and PAI antigen did not significantly differ between the three groups, but t-PA activity was elevated and PAI activity and t-PA/PAI complexes were reduced in group B. Following a maximal exercise test on the treadmill both FbDP and FgDP were significantly increased in all groups, although values for FbDP in group B and values for FgDP in group C reached a higher level than in group A. t-PA antigen and t-PA activity were also increased in the three groups. PAI activity was significantly reduced in groups A and C. t-PA/PAI complexes were significantly enhanced in all cases, but increased to a lower degree in group B. These results indicate that both aerobic and anaerobic physical conditioning induce activation of the fibrinolytic system.