Adenosine is a potent vasodilator used to study human coronary circulation. This study evaluated its effects in humans via intracoronary bolus, infusion, and intravenous infusion. Adenosine caused coronary hyperemia similar to papaverine, with maximal hyperemia achieved at 16 μg intracoronary bolus and 80 μg/min intracoronary infusion. It caused a small, brief decrease in arterial pressure and no significant changes in heart rate or electrocardiogram. Intravenous adenosine at 140 μg/kg/min caused near-maximal coronary hyperemia in most patients, with a slight decrease in arterial pressure and increase in heart rate. Adenosine was safer than papaverine or dipyridamole, with no significant effects on systemic hemodynamics or electrocardiogram. It was effective in patients with normal or abnormal coronary arteries. Adenosine's short duration of action, absence of QT interval prolongation, and efficacy via intravenous or intracoronary routes make it a safer alternative. However, it may cause atrioventricular block in some patients. Adenosine is useful for coronary flow reserve studies and 201Tl scintigraphy. It is safe for most patients, but higher doses may cause hypotension or tachycardia. Adenosine is a promising agent for coronary studies, but further research is needed to confirm its safety and efficacy in larger populations.Adenosine is a potent vasodilator used to study human coronary circulation. This study evaluated its effects in humans via intracoronary bolus, infusion, and intravenous infusion. Adenosine caused coronary hyperemia similar to papaverine, with maximal hyperemia achieved at 16 μg intracoronary bolus and 80 μg/min intracoronary infusion. It caused a small, brief decrease in arterial pressure and no significant changes in heart rate or electrocardiogram. Intravenous adenosine at 140 μg/kg/min caused near-maximal coronary hyperemia in most patients, with a slight decrease in arterial pressure and increase in heart rate. Adenosine was safer than papaverine or dipyridamole, with no significant effects on systemic hemodynamics or electrocardiogram. It was effective in patients with normal or abnormal coronary arteries. Adenosine's short duration of action, absence of QT interval prolongation, and efficacy via intravenous or intracoronary routes make it a safer alternative. However, it may cause atrioventricular block in some patients. Adenosine is useful for coronary flow reserve studies and 201Tl scintigraphy. It is safe for most patients, but higher doses may cause hypotension or tachycardia. Adenosine is a promising agent for coronary studies, but further research is needed to confirm its safety and efficacy in larger populations.