Endothelium-dependent contractions of isolated veins can be induced with arachidonic acid and thrombin. Anoxia causes endothelium-dependent contractions in a variety of isolated arteries and veins. Rapid stretch and acetylcholine contract cerebral arteries with, but not those without endothelium. Acetylcholine (and serotonin) cause endothelium-dependent contractions in the aorta of the spontaneously hypertensive, but not that of the normotensive rat.