Serotonin (5-Hydroxytryptamine, 5-HT) mediates vasoconstriction and vasodilation in the normal coronary circulation of various animal species. In the presence of coronary artery disease serotonin may inhibit coronary collateral formation and stimulate predominantly vasoconstriction. This study tested the effect of ketanserin, a selective 5-HT2 receptor antagonist and platelet aggregation inhibitor, on ischaemic myocardium blood flow and coronary collateral formation following coronary artery occlusion in primates.