The effect of neuropeptide Y (NPY) on tension development was examined in isolated canine coronary arteries, and the effects on local myocardial blood flow rate were studied in open-chest anesthetized dogs by the local 133Xe washout technique. By immunohistochemistry, numerous NPY-like immunoreactive nerve fibers were identified in the adventitia of canine coronary arteries. NPY (10(-9)-10(-6) M) supplied to isolated epicardial segments of the left anterior descending coronary artery induced a modest vasoconstriction, with a maximum tension of 0.95 mN, that was only 6.9% of the response to K+. In contrast, intracoronary NPY (0.01-10 micrograms) induced a considerable degree of vasoconstriction; the reduction of blood flow rate was dose related, with a maximum reduction to 52% of control values. The effect of intracoronary NPY (1 microgram) on maximally relaxed arterioles elicited by 30 s of ischemia was studied in separate experiments during reactive hyperemia. NPY induced a decrease in maximum blood flow during reactive hyperemia (166.6 vs. 214.6% of preocclusive blood flow rate, mean values; P = 0.05), an increase in the cumulative excess blood flow (61.0 vs. 35.3 ml/100 g; P = 0.02), and an increase in the duration of reactive hyperemia compared with control values (66 vs. 41 s; P = 0.02). Thus we conclude that in the heart NPY is a potent vasoconstrictor that seems to act preferentially on smaller intramyocardial arterioles. Furthermore, NPY inhibits vascular relaxation of myocardial resistance vessels after ischemia, suggesting that this peptide may participate in the regulation of myocardial blood flow not only during physiological conditions but also after ischemia.