This study was designed to determine whether the endothelium-derived relaxing factor induced by acetylcholine (1 μ.M) in rabbit thoracic aorta inhibits agonist-induced calcium mobilization, specifically calcium influx. Force generated in rings of rabbit thoracic aorta by norepinephrine (1 μM) was measured under isometric conditions. At the appropriate time during 1 fiM acetylcholine-induced relaxation of 1 μM norepinephrine-contracted rabbit thoracic aorta, the rings were pulse-labelled with calcium-45 to measure calcium influx. When measured in this fashion, 1 μM acetylcholine decreased the 1 μ.M norepinephrine-induced increase in calcium influx. This effect was eliminated by removal of the endothelium and by atropine (1 μM), but not by indomethacin (14 μM). Acetylcholine (1 fiM) also blocked the 60 mM potassium-chloride-induced increase in calcium influx without dramatically affecting force. The phasic contraction produced by norepinephrine (1 μM) with 2 mM lanthanum pretreatment, which is caused by release of intracellular calcium, was inhibited by acetylcholine (1 μM) in a fashion similar to 1 μM nitroglycerin. The tonic contraction produced by norepinephrine (1 μM) after depletion of the agonist-releasable pool of intracellular calcium, which is thought to be due to calcium influx, was depressed by acetylcholine (1 μM). These data suggest that endothelium-derived relaxing factor relaxes 1 μM norepinephrine-contracted rings of rabbit thoracic aorta by decreasing calcium entry and by producing an extracellular calcium-independent relaxant effect.