Intravenous acetylsalicylic acid (ASA) and magnesium (Mg) both possess antiplatelet properties and are thus potential inhibitors of the formation of arterial thrombi. Their effect on the dynamic aspects of arterial thrombus formation was investigated following intravenous administration of both substances alone and in combination. A blinded, placebo-controlled, in-vivo study was performed in 71 rats. Thrombus formation was induced by a standardized arteriotomy in the right femoral artery with inversion of the vessel wall during subsequent closure. Thrombus formation was recorded on video tapes and analysed off-line for 30 min. Animals were randomly assigned to one of four groups: 20 mg bolus of ASA followed by 0.3 mmol/h Mg (ASA/Mg group); NaCl followed by 0.3 mmol/h Mg (Mg group); 20 mg bolus of ASA followed by NaCl (ASA group); or NaCl throughout the experiment (control group). In the ASA-treated groups, serum levels of thromboxane B2 were reduced significantly, and the Mg-treated groups reached a serum level of Mg just above 2.0 mmol/l. No significant differences were observed in initial or maximum thrombus area or in mean thrombus area during the study period. In the ASA/Mg group, a trend towards reduced thrombus formation was observed (P = 0.06). In the same group, seven of 22 animals developed an occlusive thrombus (P < 0.01), an unexpected adverse event possibly related to the combined administration of ASA and Mg.