The effect of oral contraceptives on a number of plasma fibrinolytic variables and antithrombin III was studied in 20 highly trained women before and immediately after exhaustive exercise on a bicycle ergometer. Two groups were studied, 11 women using an oral contraceptive agent (OCA group) and 9 women not using an oral contraceptive (control group). Factor XII, prekallikrein, plasminogen, alpha 2-antiplasmin, and antithrombin III activities were determined in plasma as well as a number of cardiovascular, respiratory, and metabolic components. Resting values showed a significantly higher level of prekallikrein and plasminogen in the OCA group compared with the control group. Exercise plasma values of components of the fibrinolytic system were significantly higher than resting values in both groups. However, when corrected for plasma volume changes, the exercise levels of prekallikrein and antithrombin III were significantly decreased in the OCA group only, whereas alpha 2-antiplasmin exercise level was significantly lower in both groups. Our results indicate that use of OCA even with an estrogen dosage as low as 30 micrograms has an effect on resting and exercise fibrinolytic variables; an effect, however, which does not seem to interfere with maximal performance in trained women.