The contracting and relaxing effects of purines and UTP were investigated on rings of mouse thoracic aorta in vitro. UTP, ATPγS, and α-β-Methyleneadenosine 5' triphosphate contracted rings with and without endothelium. On the contrary, adenosine, AMP, ADP, ATP, and 2-(methylthio) adenosine 5'-diphosphate had no effect on relaxed rings. When rings were tonically contracted by U46619 a thromboxane A2 analogue, ATP, ADP, ATPγS, 2-(methylthio) adenosine 5'-diphosphate, and UTP caused endothelium-dependent but not independent relaxations. I conclude that ATP acts on P2Y₂ and P2Y₁ receptors on the endothelial cells to cause endothelium-dependent relaxation. In this tissue, the relaxing effect of ATP dominates by endothelium-dependent ways when aorta rings are contracted by a stable thromboxane A2 analog. However receptors mediating contraction in response to purines and pyrimidines are present on smooth muscle cells. Indeed, the stimulation of P2Y receptors by UTP as well as the activation of P2X family receptors by ATPγS causes a contraction. The potential contractile effect of ATP seems masked by its hydrolysis by ectonucleotidases.