1. The contraction induced by clonidine and guanfacine but not phenylephrine was enhanced in endothelium-denuded and methylene blue pretreated rabbit aortic, pulmonary artery rings and isolated perfused whole superior mesenteric, carotid and femoral arteries from the same species. 2. The responses to acetylcholine in guanfacine preconstricted superior mesenteric and carotid arteries were also augmented when compared with phenylephrine and clonidine preconstricted segments. No difference was observed to the relaxing activity of acetylcholine in the aortic, pulmonary artery rings and whole perfused femoral artery contracted by phenylephrine, clonidine and guanfacine. 3. Acetylcholine also produced a biphasic effects in the pulmonary artery rings precontracted with the used alpha-adrenoceptor agonists. The contractile effect was observed with the concentration up to 10(-6) M of acetylcholine and was higher for guanfacine than phenylephrine and clonidine precontracted rings. 4. These results were taken as an evidence for the specificity of alpha 2-adrenoceptor agonists on the production and release of EDRF from vascular endothelium and in this respect guanfacine seems to have higher potency than clonidine. 5. These results also indicate that production or release of EDRF from vascular endothelium may vary depending on the regional vascular bed.