Acetylsalicylic acid (ASA) was tested for its antithrombotic activity in the arterial system after prophylactic administration to rats, using a new standardized method. Damage of the vessel wall was produced by chilling a small segment of the left carotid artery. Dose related, significant results were obtained after 3 mg/kg orally. If higher doses (10 and 30 mg/kg) are administered, the formation of non-occlusive thrombi is inhibited by 70--90% on the basis of thrombus weight. As the frequency distributions show, there are significantly more zero-values in the ASA treated groups (total 50%) than in the control groups. However, the incidence of occlusive thrombi was not changed by ASA. The long-lasting effect of ASA in inhibition of platelet aggregation was confirmed. The formation of arterial thrombi is significantly inhibited after prophylactic administration of 30 and 10 mg/kg up to 48 h before initiation of thrombosis. After administration of 3 mg/kg orally, only insignificant effects were observed. Thus the duration of action depends on the dose used.