The relative efficacy and sequelae of several coronary vasodilator methods were compared in dogs. Maximal coronary vasodilation with least apparent side effect was achieved with infusion of adenosine triphosphate (ATP) at the rate of approximately 600 microng/minute (approximately 20 microng/kg/minute). This method increased conductance to 7 to 8 times control levels and may have fully depleted vasodilator reserve. Bolus injections of ATP or papaverine alone increased conductances three to four fold and approximated the effects of 15 seconds of occlusion of the circumflex coronary artery. Duration of effect was variable. Diatrizoate injections were somewhat less effective vasodilators and produced greater electrocardiographic and systemic effects. Bolus injections of mixtures of ATP and papaverine demonstrated additive effects, and increased conductance 5 to 6 times control levels. In further exploring the diagnostic value of vasodilatory redistribution of myocardial blood flow or appearance of collateral circulation as a sign of a hemodynamically significant stenosis, selective intracoronary infusion of ATP is a promising vasodilatory method.