We examined the endothelial dependence of responses to ACh and some vasodilator drugs by using the central tail artery of the rat perfused with Krebs buffer. Perfusion with ACh (100 nM-100 microM) produced dose-dependent vasodilatation of arteries preconstricted with norepinephrine and antagonized pressor responses to periarterial electrical stimulation. Endothelium was removed by introducing a fine catheter through the lumen or a stream of gas (O2 95%-CO2 5%) intraluminally. Both procedures prevented the vasodilator effect of ACh. Gassing also abolished the vasodilatation in response to hydralazine 334 nM but not to equidilator amounts of papaverine 13 microM, or nitroglycerin 50 nM. These results indicating endothelial dependence of hydralazine and ACh responses are in accord with our previous studies on vascular rings.