In the present study we examined whether the vasoconstrictor response to hyperoxia is through activated adrenergic function (Protocol 1), or through inhibitory effects on nitric oxide synthase (NOS) and/or cyclooxygenase (COX) (Protocol 2). We also tested whether any such effects are altered by body heating. In protocol 1 (n=11 male subjects), release of norepinephrine from adrenergic terminals was blocked locally by iontophoresis of bretylium (BT). In protocol 2, the NOS inhibitor N^G-nitro-L-arginine methyl ester (L-NAME) and the nonselective COX antagonist Ketorolac (Keto) were separately administered by intradermal microdialysis in 11 male subjects. In the two protocols, subjects breathed 21% (room air) or 100% O₂ in both normothermia and hyperthermia. Skin blood flow (SkBF) was monitored by laser-Doppler flowmetry. Cutaneous vascular conductance (CVC) was calculated as the ratio of SkBF to blood pressure. Protocol 1: Breathing 100% O₂ decreased (P<0.05) CVC at the BT-treated and at untreated sites from the levels of CVC during 21% O₂ breathing both in normothermia and hyperthermia. Protocol 2: The administration of L-NAME inhibited (P<0.05) the reduction of CVC during 100% O₂ breathing in both thermal conditions. The administration of Keto inhibited (P<0.05) the reduction of CVC during 100% O₂ breathing in hyperthermia but not in normothermia. These results suggest that skin vasoconstriction with hyperoxia is partly due to the decreased activity of functional NOS in normothermia and hyperthermia. We found no significant role for adrenergic mechanisms in hyperoxic vasoconstriction. Decreased production of vasodilator prostaglandins may play a role in hyperoxia-induced cutaneous vasoconstriction in heat-stressed humans.