Norepinephrine (NE) activates isolated coronary conduit arteries by stimulating Ca2+uptake into the smooth muscle cells. Blockade of Ca2+influx by removal of Ca2+from the bathing medium or addition of 10 miu LaCl3prevents the NE-induced contraction in the dog coronary artery but still allows NE to induce a rapid transient contraction in the rabbit aorta. Under these conditions, NE stimulates45Ca2+efflux from rabbit aorta but fails to do so in the coronary artery. The difference in behaviour between the two arteries is attributed to the presence of an intracellular NE-sensitive Ca2+fraction in the rabbit aorta and its absence from the dog coronary artery. This difference also explains the much greater sensitivity of the NE-induced contractions in the dog coronary to the relaxant effects of the Ca2+antagonists, D600 and SKF525A, than that seen in the rabbit aorta. High K+-induced contractions of both the coronary artery and the aorta are equally sensitive to the Ca2+antagonists. Circ Res 46: 426-429, 1980