Background: In recent decades, the prevention of vascular calcification (VC) by pyrophosphate (PPi), bisphosphonates, and polyphosphates has been extensively reported. However, the possibility of direct inhibition of calcium phosphate deposition (CPD) by nucleoside-associated polyphosphates has not been addressed. We analyzed the role of ATP as an inhibitor of calcification in 2 ways: by characterizing the extracellular hydrolysis of ATP as source of PPi in the aorta, and by demonstrating the ability of ATP to prevent CPD by acting as a polyphosphate. Methods and Results: In our study, both PPi and ATP hydrolysis in the rat aorta was kinetically characterized, thereby resulting in apparent Michaelis-Menten constants of 179 and 435μmol/l, respectively, with the corresponding maximal velocities of 55.1 and 6,177nmol·g^–1·min^–1. According to these kinetic parameters, the theoretical PPi concentration in the aortic wall was 0.4–3.5μmol/L (for an ATP concentration range of 0.1–1.0μmol/L). In addition, we showed that nonhydrolyzable molecules are more efficient as CPD inhibitors than endogenous compounds, in accordance with the IC₅₀ values: 1.2–2.4μmol/L for bisphosphonates vs. 8.8μmol/L for PPi, and 0.5–1.5μmol/L for nonhydrolyzable ATP analogs vs. 3.2μmol/L for ATP. Conclusions: Extracellular ATP can play an important role in the prevention of VC, not only as the source of PPi but also as a direct inhibitor of CPD.  (Circ J 2013; 77: 2145–2151)