The vasodilator effect of caffeine in animal models arteries has been demonstrated previously. However, studies with the same methodology using human arteries in vitro have not been performed.The in vitro vasoactive effects of caffeine was evaluated on human internal mammary arteries.Internal mammary artery rings were used (n = 20). Endothelial function was evaluated with acetylcholine at a concentration of 3.16 x 10 -6 M, nitroglycerine at cumulative concentrations of 10 -11 M to 10 -4 M and caffeine with cumulative concentrations of 10 -8 M to 10 -4 M.Nitroglycerin produced a maximum relaxation percentage of 87.4 +/- 12.3%, caffeine a percentage of 86.9 +/- 21.0% in arteries with functional endothelium, and of 71.6 +/- 28.6% in arteries with endothelial dysfunction. No differences were detected among the three groups ( p=0.289). Similarly, no differences were found between EC 50 in arteries with functional endothelium (1.66 x 10 -5 +/-1.57 x 10 -5 M) and dysfunctional arteries (7.8 x 10 -5 +/-14.6 x 10 -5 M). Nitroglycerine proved more potent than caffeine (EC 50 = 4.3 x 10 -9 +/-4.4 x 10 -9 M) ( p=0.013).Although nitroglycerin was a more potent vasodilator, caffeine had a strong arterial vasodilator effect regardless of endothelial function in human arteries.