In the systemic vasculature, hypoxia elicits a local vasodilator response that may be partially mediated by ionic channels on vascular smooth muscle, such as adenosine triphosphate sensitive K<sup>+</sup> channels. Recent electrophysiological studies suggest that hypoxia may also inhibit voltage-dependent Ca<sup>2+</sup> channels (L type) on peripheral vascular smooth muscle cells. We hypothesized that hypoxia elicits relaxation of vascular smooth muscle by inhibiting L-type Ca<sup>2+</sup> channels. In endothelium-denuded rat thoracic aortic rings contracted with phenylephrine, mild and moderate hypoxia (PO<sub>2</sub> 35 and 20 mm Hg, respectively) elicited significant relaxation. Pretreatment with the L-type Ca<sup>2+</sup> channel antagonist nifedipine completely inhibited mild hypoxic relaxation and diminished relaxation under moderate hypoxia, whereas glibenclamide, a blocker of adenosine triphosphate sensitive potassium channels, only attenuated the response to moderate hypoxia. In rings contracted with the L-type Ca<sup>2+</sup> channel agonist (–)BAY K 8644 both mild and moderate hypoxia elicited almost complete relaxation. Furthermore, in rings contracted with hyperkalemic solutions (85 m M K<sup>+</sup> or 120 m M K), mild and moderate hypoxia elicited significant relaxations. Thus, we conclude that hypoxia acts directly on vascular smooth muscle to cause relaxation in part by inhibiting L-type Ca<sup>2+</sup> channels.