Endothelium-derived nitric oxide (EDNO) contributes to basal systemic vascular resistance under normoxic conditions. The purpose of this investigation was to determine whether EDNO contributes to the regulation of limb vascular resistance during hypoxia in healthy humans. Forearm blood flow was assessed by venous occlusion plethysmography. Hypoxia was induced by delivering a mixture of N2 and O2 via a gas blender adjusted to reduce the PO2 to 50 mmHg. During hypoxia, forearm blood flow increased from 2.4 +/- 0.2 to 3.0 +/- 0.3 ml.100 ml-1.min-1 (P < 0.001), and forearm vascular resistance decreased from 38 +/- 3 to 29 +/- 3 units (P < 0.001). The nitric oxide synthase inhibitor NG-monomethyl-L-arginine (L-NMMA, 2,000 micrograms/min intra-arterially) was administered to eight subjects. The percent increase in forearm vascular resistance after administration of L-NMMA was greater during hypoxia than normoxia (67 +/- 14 vs. 39 +/- 15%, P < 0.05). L-NMMA reduced the forearm vasodilator response to hypoxia from 27 +/- 3 to 11 +/- 5% (P = 0.01). To exclude the possibility that this attenuated response to hypoxia was a consequence of vasoconstriction and not specific for nitric oxide synthase inhibition, six subjects received intra-arterial phenylephrine. Phenylephrine did not affect the vasodilator response to hypoxia (17 +/- 3 vs. 21 +/- 6%, P = NS). It is concluded that EDNO contributes to hypoxia-induced vasodilation in the forearm resistance vessels in healthy humans.