1. The pharmacological properties of a vasodilating purine-activated receptor that is not a P1 or P2-purinoceptor were investigated. 2. In rabbit isolated thoracic aorta precontracted with noradrenaline, ATP induced a 50% relaxation at 0.25 mM (EC 50%); in the absence of endothelium, EC 50% was 2.5 mM. 3. Adenosine induced a relaxation that was not different in both the presence and absence of endothelium, being EC 50% 0.48 and 0.37 mM, respectively. 4. The potent and selective P2y-purinoceptor agonist 2-methylthio-ATP (0.03-10 microM) induced a relaxation only in the presence of endothelium. 5. In de-endothelialized aorta, 8-phenyltheophilline (8-PT: P1 antagonist) and 3,7-dimethyl-1-propargylxanthine (DMPX: A2 antagonist) did not antagonize ATP- and adenosine-induced relaxation. 6. The present data support the presence of a new site of action for purines in rabbit isolated thoracic aorta. 7. A P3 subtype of purinoceptor, that may be identified in the hypothesized "nucleotide" receptor, is proposed.