An increased formation of nitric oxide (NO) and calcitonin gene-related peptide (CGRP) by the human brain in hypoxia may be implicated in the pathophysiology of acute mountain sickness (AMS) through direct activation of the trigeminovascular system. To test this hypothesis, we examined eleven healthy males aged 27 (mean) ± 4 (SD) years in normoxia (21.0%O₂) and following 9h passive exposure to normobaric hypoxia (12.9%O₂). Symptoms of AMS were recorded using the Lake Louise (LL) and Environmental Symptoms Questionnaire-Cerebral Symptoms (ESQ-C) scoring systems and headache determined using a visual analogue scale. Blood samples were obtained simultaneously from the radial artery and right internal jugular vein and assayed for plasma nitrite (NO₂^–) using ozone chemiluminescence and CGRP via radio-immunoassay. Global cerebral blood flow (CBF) was determined in the desaturation mode using inhaled nitrous oxide (5%) as the tracer (Kety and Schmidt, 1945). Cerebral plasma flow (CPF) was calculated as CBF x (1-haematocrit) and net flux determined as CPF x arterio-venous concentration difference (a-v_diff). Hypoxia increased the LL (3.0 ± 1.9 vs. 0 ± 0 points, P < 0.05 vs. normoxia), ESQ-C (0.730 ± 0.683 vs. 0.000 ± 0.000 points, P < 0.05), and headache (20 ± 18 vs. 0 ± 0 mm, P < 0.05) scores (paired samples t-tests). CBF changed from 85 ± 15 to 96 ± 17 mL/100g/min in hypoxia (P = 0.09). Normoxia was associated with a positive a-v_diff for NO₂^– (consistent with an influx) that was blunted by hypoxia due primarily to a reduction in arterial inflow (Table 1). In contrast, hypoxia affected neither arterial CGRP nor cerebral exchange and no relationships were observed between the change (hypoxia-normoxia) in the net flux of NO₂^– or CGRP respectively and LL (r = -0.14/-0.10), ESQ-C (r = -0.35/-0.23) or VAS (r = 0.33/-0.11) scores (P > 0.05, Pearson Product Moment Correlation). In conclusion, our findings do not support a role for increased cerebral formation of NO₂^– and CGRP as molecular risk factors for AMS. On the contrary, hypoxia blunted the cerebral uptake of NO₂^–. Whether this reflects decreased consumption subsequent to a free radical-mediated reduction in systemic NO bioavailability and/or PO₂-driven re-apportionment of NO is the focus of current attention.