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To investigate the mechanism of lidocaine’s effect to cause vasorelaxation, swine epicardial mid-right coronary arterial rings were placed under constant (5 g) tension in a muscle bath, precontracted with 35 mmol/l KCl and exposed to increasing concentrations of lidocaine (3–2,000 μg/ml). At a concentration of 10μg/ml, mild vasoconstriction occurred, increasing tension 1.9 ± 0.1 % above baseline. Vasodilation began to occur at 30 μg/ml and was maximal at 2,000 μg/ml, reducing tension 97.5 ± 0.2% below baseline. Vasodilation was not altered significantly by removal of endothelium or by pretreatment with propranolol or indometacin.