Drugs with possible antithrombotic properties have to be investigated in models for inducing experimentalthrombosis. Some of the well known antiplatelet drugs and anticoagulants were studied in the venous (4 models, 370 experiments) and arterial system (2 models,200 experiments). Thrombi were produced by different endothelial lesions, by stasis and hypercoagulability or a combination of these thrombogenic factors. In the venous system heparin reduced the thrombus size in comparison with controls by 98% (significantly, p<0.05 = s), phenprocoumon 91% (s), acetylsalicylic acid (ASA) 30%, ASA and dipyridamole (DIPY) 69%, carbenicillin 54%. In the arterial system a minor effect could be seen: Carbenicillin 72 %(s), ASA and DIPY 36%, ASA 8%, heparin 52%, phenprocoumon 17 %. Dependent on the experimental conditions ASA may demonstrate venous thrombus size reductions by 0, 6 or 30% and heparin by 33,98,81,68 or 88%, respectively. Comment: A drug may demonstrate an effect in one model but not necessarily in another one. The antithrombotic efficacy of new substances has to be investigated in models characterized by their thrombogenic conditions and thrombus structure and compared with that of well known antithrombotic substances.