We studied the effects of exercise and physical training on coagulation parameters and fibrinolytic activity in 16 sedentary non-insulin-dependent diabetics and nine control subjects matched for prior physical activity. Parameters were measured at rest and after 30 minutes of bicycle exercise at 70% to 75% of maximal oxygen uptake before and after 6 weeks of thrice-weekly physical training. In the untrained state, fibrinolytic activity was impaired in diabetics compared with controls (1.26 +/- 0.19 v 2.20 +/- 0.34 U; P less than .03), and resting levels of plasma fibrinogen (329 +/- 21 v 266 +/- 17 mg/dL; P less than .01) and the prothrombin time (PT) maximal velocity (Vmax) (4.9 +/- 0.5 v 2.9 +/- 0.5; P less than .05) were increased. The activated partial thromboplastin time (APTT) Vmax was also increased but this did not reach statistical significance (3.6 +/- 0.2 v 2.3 +/- 0.5; P less than 0.10). Activation of fibrinolysis occurred following exercise in both groups but the peak activity and increment were less in diabetics. Physical training for 6 weeks had no effect on plasma fibrinogen levels but significantly improved the resting and postexercise APTT Vmax and resting fibrinolytic activity in diabetics. The exercise-induced increment in fibrinolytic activity following training remained depressed compared with normal controls. The changes in APTT Vmax correlated with changes in the indices of blood glucose control. The relevance of these findings to possible antiatherogenic effects of exercise and the mechanism by which exercise produces these effects remain to be established.