The alpha adrenoceptor properties of the rabbit ear artery and thoracic aorta were assessed using isolated blood vessel rings mounted in tissue baths. ED50 values and dissociation constants for norepinephrine (NE), epinephrine (EPI), isoproterenol (ISO), methoxamine (MET) and clonidine (CLO) were determined by analysis of dose-response data with and without partial inactivation of alpha receptors by phenoxybenzamine. The order of affinity of these agonists for the alpha receptors of ear artery was CLO less than EPI = NE = MET much less than ISO, and that in aorta was EPI less than NE less than CLO less than MET less than ISO. Alpha receptor reserves were larger in ear artery than aorta by factors of 182, 17, 7, 3.8 and 2 for EPI, NE, ISO, MET and CLO, respectively. In contrast, the fraction of receptors occupied at threshold contractile response was larger in aorta than ear artery with every agonist studied except CLO. The maximal response to the partial agonist, CLO, expressed as a fraction of the maximal response to NE in each vessel, was nearly 1.8 times greater in ear artery than in aorta. It is concluded that the alpha receptors of rabbit ear artery are qualitatively different from those of the thoracic aorta. In addition, the larger receptor reserves, smaller fraction of receptors occupied at threshold and the larger maximal response of CLO in ear artery support the hypothesis that alpha receptor density in this vessel is greater than that in the aorta.