Some properties of the receptor population mediating contraction to I-norepinephrine (NE) were studied in rabbit aorta and the intrapulmonary and basilar arteries. The constrictor effect of NE in isolated vessel ring segments were recorded, both in the absence and presence of concentrations of phenoxybenzamine which depressed the maximal contractions to NE. Dose-response curves to NE were biphasic in the basilar artery. In the basilar artery and in medium-sized intrapulmonary arteries, the response to maximally effective concentrations of NE were always lower than responses to other agents. In the larger intrapulmonary artery segments and aorta the responses to NE and serotonin were of similar magnitude. The ED50 for NE did not change with vessel size in intrapulmonary arteries, whereas that of the basilar artery was considerably greater than in the pulmonary and aorta segments. The NE dissociation constant (KA) and the receptor occupation-response relationship was calculated in all vessel segments. The KA for NE was similar in part I of the basilar artery, aorta and both sizes of pulmonary artery. The KA for NE in part II of the basilar artery dose-response curves was 8 to 10 times greater. Half-maximal responses by NE were produced by occupation of 10% of receptors in the aorta and the large intrapulmonary artery and of 40 to 50% receptors in the basilar and medium-sized intrapulmonary artery. It is concluded that in some smaller arteries the small NE response may be limited by receptor number, and that whereas this may not be a phenomenon common to all vessels of this size, it may reflect specialization of certain vascular beds.