Experiments were designed to determine the role of the endothelial cells and the metabolism of arachidonic acid in anoxic contractions of isolated canine basilar arteries. Rings, with and without endothelium, of these arteries were suspended for isometric tension recording; anoxia was induced by switching the mixture gassing the organ chamber from 95% O2-5% CO2 to 95% N2-5% CO2. In rings with endothelium, anoxia evoked increases in tension under basal conditions and during contractions to 5-hydroxytryptamine, uridine triphosphate, prostaglandin F2 alpha, and high K+. Under control conditions, these anoxic contractions were not prevented by alpha-adrenergic and serotonergic antagonists, by apyrase, or by inhibitors of cyclooxygenase. Anoxia prevented endothelium-dependent relaxations evoked by vasopressin and thrombin. In rings without endothelium, anoxia caused increases in tension during contractions evoked by various agonists, and in unstimulated preparations after inhibition of cyclooxygenase. Anoxic contractions were abolished by calcium entry blockers. These observations suggest that anoxic contractions of isolated canine basilar artery can be explained by the release of endothelium-derived contracting factor(s) and the accelerated entry of calcium in the smooth muscle cells, which possibly results from a diversion of arachidonic acid from the cyclooxygenase to the lipoxygenase pathway.