Using a combination of exercise and heat stress, the present study examined whether prior disruption of bloodbrain barrier (BBB) integrity in normoxia would alter individual susceptibility to neurovascular headache initiated during subsequent exposure to inspiratory hypoxia. Eleven males aged 24 ± 2 years (mean ± S.D.) were randomly assigned to complete two trials separated by 4 weeks recovery. During the experimental trial (EXP), subjects rested in normoxia (PRE-N) before being immersed supine in water (39°C) for 30min followed by 45min of cycling (35°C) at 60% of their previously determined peak oxygen uptake (PRIMER-N). Subjects were then exposed to normobaric hypoxia (12% O₂ at 25°C) where they rested for 8h (POST-H). The control trial (CON) incorporated the hypoxic exposure without prior immersion or exercise. Headache scores were assessed using a clinically validated visual analog scale. Venous blood samples were mixed ex vivo with ±-phenyl-tert-butylnitrone (PBN) prior to X-band electron paramagnetic resonance spectroscopy. Additional samples were assayed for S100², neuron specific enolase (NSE), myoglobin and endotoxin according to established techniques following correction for plasma volume shifts (Bailey et al. 2005). Exercise-heat stress increased the serum concentration of S100² and PBN-adducts (Table 1), whereas no changes were observed in NSE, myoglobin or plasma endotoxin. These increases were compounded during subsequent exposure to hypoxia and were associated with a marked increase in the severity and incidence of hypoxic headache (HH). In conclusion, exercise-heat stress increased BBB permeability and free radical generation independent of neuronal or sarcolemmal membrane damage. The increased susceptibility to HH suggests that this condition may have a vasogenic basis.