1. Reversible contraction of canine basilar artery, produced by hypoxia, persisted after mechanical and chemical removal of the endothelium. The removal of endothelium was confirmed by scanning electron microscopy as well as by the abolition or reversal of the relaxant response to acetylcholine or arginine8-vasopressin. 2. Hydroquinone, believed to block selectively endothelium-mediated relaxation, also preferentially attenuated hypoxic contractions even in the absence of endothelium but did not reduce responses to 5-hydroxytryptamine (5-HT) or high external potassium. 3. Contractions induced by red blood cell haemolysate, which occur independently of the endothelium, were also selectively attenuated by hydroquinone. 4. Contractions caused by hypoxia were inhibited by pretreatment with adenosine or by its application after contraction had developed. 5. Hypoxic contraction in canine basilar artery may result partly from a direct effect on smooth muscle as well as through the endothelium. 6. Hydroquinone may have an additional locus of action in smooth muscle cells besides its well known effect on the endothelium.