Previously we showed in normoxic (N) rats that vasodilatation evoked by 5 min acute hypoxia (8%O2) is reduced by the xanthine oxidase (XO) inhibitor oxypurinol and potentiated by infusion of superoxide dismutase (SOD)1. 50% of the vasodilatation evoked by 8%O2 is mediated by adenosine2 so we proposed that during hypoxia, H2O2 produced by SOD from O2- generated from the breakdown of adenosine by XO, contributes to vasodilatation. As chronic hypoxia (CH) increases oxidative stress we investigated the role of this pathway in CH rats. CH male Wistar rats breathed 12%O2 for 1, 3 and 7 days prior to acute experiments. In anaesthesized (Alfaxan 12mg.kg-1.hr-1) normoxic (N) and 1, 3 and 7CH rats, integrated femoral vascular conductance (IntFVC) was measured 5 min before and during 8%O2. In Group 1 (n=10, 7, 10 & 9) the response to 8%O2 was measured before and after the XO inhibitor oxypurinol and in Group 2 (n=9 each) the SOD inhibitor DETC. In a further group (n=6 each) of N, 1, 3 and 7CH rats XO and SOD activity was assayed in Tibialis Anterior (TA) muscle. In N rats, XO inhibition increased baseline IntFVC (3.0±0.2 vs 3.3±0.2 CU*; mean±SEM; *P<0.05 Student’s paired t test) but decreased the response to 8%O2 (3.4±0.4 vs 2.6±0.3 CU*). In contrast, in CH rats, oxypurinol had no effect on baseline IntFVC or the response to 8%O2. XO activity was comparable in CH and N TA. In N rats, SOD inhibition increased baseline IntFVC (1.9±0.2 vs 2.5±0.2 CU*) but had no effect on the response to 8%O2. However, in CH rats, DETC had no effect on baselines but reduced the IntFVC response to 8%O2 (1CH 1.9±0.2 vs 0.9±0.2*; 3CH 1.8±0.2 vs 0.9±0.3*; 7CH 1.9±0.2 vs 0.9±0.4 CU*). Further, in TA, SOD activity was decreased by CH (N 669±24 vs 1CH 454± 18*; 3CH 390±28*; 7CH 535±16* U.ml-1.mg protein-1). In contrast to N rats, XO inhibition with oxypurinol had no effect on the vasodilatation evoked by 8%O2 in CH rats suggesting that O2- produced from the breakdown of adenosine no longer contributes to vasodilatation via the production of H2O2 by SOD. However, inhibition of SOD reduced the vasodilatation evoked by 8%O2 in CH rats suggesting that O2-, from sources other than adenosine breakdown by XO, can be dismuted to H2O2. Other sources of O2-, which may be important in hypoxia, include Complexes I and III of the mitochondrial respiratory chain, NADPH oxidase and metabolism of arachidonic acid by cyclooxygenase. In addition, the change in the contribution of H2O2 to vasodilatation evoked by 8%O2 in CH rats cannot be explained by a change in SOD activity. It may be that measuring activity in whole muscle homogenates masks any changes in SOD activity in the vasculature. Further studies will look at the effect of CH on SOD expression and activity specifically in the vasculature.