The effects of a 10-week physical-conditioning program on fibrinolytic activity at rest and after stimulation by venous occlusion were studied in 69 healthy adults 25 to 69 years old. Physical conditioning was documented by treadmill performance, and fibrinolysis was measured with a newly developed radioenzymatic assay. Whereas fibrinolysis declined at rest from 16.2 +/- 1.3 to 11.4 +/- 0.8 units (mean +/- S.E.M.) (P = 0.0017), the increment in fibrinolysis produced by venous occlusion was increased from 21.7 +/- 2.9 to 33.8 +/- 4.7 units (P = 0.0037). This augmentation was most marked in women, persons with low initial levels of stimulated fibrinolysis, and persons with low initial physical fitness. We conclude that physical conditioning can enhance the augmentation of fibrinolytic activity that occurs in response to venous occlusion. Enhanced fibrinolysis in response to thrombotic stimuli could be an important mechanism in the beneficial effect of habitual physical activity on the risk of cardiovascular disease.