OBJECTIVE This study was performed to see whether ethanol could potentiate the inhibitory effect of aspirin on recurrent platelet thrombus formation and subsequent embolisation, which have been shown to cause cyclic reductions in blood flow in 70% of mechanically stenosed dog coronary arteries. METHODS After mechanical stenosis of the circumflex coronary artery in open chest dogs, cyclic variations in blood flow were measured during treatment with aspirin, ethanol, or both. METHODS 20 adult mongrel dogs were used: in group 1 (n = 5) each dog was given aspirin 5 mg.kg-1; in group 2 (n = 5) each was given 1.2 ml.kg-1 ethanol; in group 3 (n = 10) each was given aspirin 1 mg.kg-1 combined with either 0.3 or 0.6 ml.kg-1 ethanol. RESULTS In group 1, aspirin 5 mg.kg-1 abolished cyclic reductions in circumflex blood flow, but an intravenous infusion of adrenaline (0.4 microgram.kg-1.min-1 for 15 min) caused them to return. In group 2, ethanol 1.2 ml.kg-1 abolished cyclic flow reductions but they returned after adrenaline infusion challenge. In group 3, aspirin 1 mg.kg-1 combined with either 0.6 or 0.3 mg.kg-1 ethanol abolished cyclic flow reductions and protected against their return during adrenaline infusion challenge in 9/10 dogs. CONCLUSIONS The results suggest that ethanol potentiates the inhibitory effects of aspirin on platelet thrombus formation in our model of simulated coronary artery disease.