This study was carried oat to examine the effects of atrial natriuretic peptide on coronary collateral blood flow. Studies were performed in nine adult mongrel dogs 3.4 months after embolic occlusion of the left anterior descending coronary artery had been performed to stimulate collateral vessel growth. At the time of study the anterior descending coronary artery was cannulated to allow estimation of interarterial collateral flow from measurements of retrograde blood flow. Injection of radioactive microspheres during retrograde flow collection allowed simultaneous determination of continuing tissue flow for evaluation of microvascular collateral communications. Atrial natriuretic peptide in doses of 20 and 200 μg administered into the left atrium resulted in 17 ± 3.0% and 34 ± 4.5% increases in retrograde flow, respectively (each<0.01). Tissue flow in the collateral dependent myocardial region did not change in response to atrial natriuretic peptide. After the larger dose of atrial natriuretic peptide, the administration of nitroglycerin (10 μg/kg into the left atrium) caused no further increase of retrograde blood flow, and no further decrease of collateral vascular resistance. These data indicate that atrial natriuretic peptide causes vasodilation of moderately welldeveloped interarterial coronary collateral vessels. 1671 1678