In 178 patients undergoing hysterectomy, studies on heparin level in plasma (Coatest Kit Kabi) were carried out before and after surgery. Low dose heparin (LDH) and the combination LDH/dihydroergotamin (DHE) were administered 12-hourly (5000 I heparin) beginning 2 hours preop .. The controls were given acenocoumarol postop .. 36 patients were examined 3-hourly on the day of the operation. In 71 patients the I-125-fibrinogen-uptake-test for diagnosis of DVT was performed and blood sampling was carried out from day 0 (preop) daily until day 8. In 15 patients the fibrinoge test was positive. In addition heparin levels were studied daily from day 0 to 8 i 2 groups of 56 and 17 patients. Blood samples were taken 11-12 hrs and 7-8 hrs res after the last injection of LDH or LDH/DHE. On the day of surgery heparin levels had risen 3 hours after the first injection of LDH and LDH/DHE from 0 . 035 to 0.065 IU / ml resp .. After 9 hours in the LDH/DHE group the average values (0.043) were still statistically distinguishable from the original ones. In both groups heparin showe d a significant decrease on the first postoperative day, which was more marke in patients with positive J -fibrinogen-test. On the following days the heparin levels returned to the original values and remained at this level up to day 8. There were no differences between prophylaxis with LDH and the combination of LDH and DHE in the group in which blood samples were taken 7-8 hrs after the last injection there was no accumulation of heparin in either groups.