Forty women undergoing surgery under general anaesthesia for hyperplasia mammae were randomized to treatment with low dose heparin (5000 IU twice daily) or not. Preoperatively, and repeated on the 3. postoperative day, assays of euglobulin clot lysis time (ELT) after venous stasis, tissue plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI) were performed. Compared to the control group heparin was found to give a significant rise in t-PA antigen before (24.0 vs. 11.2 ng/ml, p = 0.02), and especially after venous stasis (104.8 vs. 47.3 ng/ml, P = 0.007). t-PA activity was also significantly more increased after venous stasis in the heparin group than among the controls (4.2 vs. 1.4 U/ml, p = 0.04). This was also reflected in the ELT after venous stasis which was significantly shorter in the heparin group (p = 0.01). No differences in PAI were found between the groups. The present results point to a heparin-induced increase of t-PA synthesis in the endothelium, also giving rise to an increased level of circulating t-PA as measured immunologically. This effect of small dose heparin may play an important role in the prophylaxis against thrombo-emboli, in addition to the anticoagulant effect.