Cyclic adenosine diphosphate ribose and adenosine diphosphate ribose (ADPR) play an important role in the regulation of intracellular Ca<sup>2+</sup> release and K<sup>+</sup> channel activity in the coronary arterial smooth muscle. The role of these signaling nucleotides in the control of vascular tone has yet to be determined. The present study was designed to determine whether ADPR produces vasodilation in coronary arteries and to explore the mechanism of action of ADPR. ADPR (10–60 µmol/l) was found to produce endothelium-independent relaxation in a concentration-dependent manner in isolated and pressurized small bovine coronary arteries. The ADPR-induced vasodilation was substantially attenuated by adenosine deaminase (0.2 U/ml), and the P<sub>1</sub> purinoceptor antagonist 8-( p-sulfophenyl)theophylline (50 µmol/l), with maximal inhibitions of 60 and 80%, respectively. When the coronary arterial homogenates were incubated with ADPR, the production of adenosine and 5′-AMP was detected. The adenosine production was blocked by the 5′-nucleotidase inhibitor, α,β-methylene adenosine 5′-diphosphate (MADP, 1 mmol/l), which was accompanied by a corresponding accumulation of 5′-AMP. This 5′-AMP accumulation was substantially inhibited by the apyrase inhibitor sodium azide (10 mmol/l). Moreover, ADPR was hydrolyzed into 5′-AMP by purified apyrase. In agreement with their inhibitory effect on the adenosine production, MADP and sodium azide significantly attenuated the vasodilator response to ADPR. The metabolism of ADPR to adenosine was only detected in cultured coronary arterial smooth muscle cells but not in endothelial cells. We concluded that ADPR produces vasodilation in small coronary arteries and that the action of ADPR is associated with the adenosine production via an apyrase- and 5′-nucleotidase-mediated metabolism.