Oxidation of hypoxanthine (HX) by xanthine oxidase (XO) has been hypothesized to be a potential source of oxygen-derived free radicles (O2−) during reoxygenation-perfusion of hypoxic-ischemic organs. Allopurinol (Al), a XO inhibitor, can reduce reperfusion injury. During hypoxia-ischemia, components of the mitochondrial electron-transport chain become reduced. With reoxygenation-reperfusion O2− are generated. A Near Infrared Spectroscope (Niroscope) was used to continuously monitor changes in cytochrome aa3 (Cytaa3), blood volume (BV), saturated Hb (HbO) and desaturated (HB-) in 24 rats. The rats were divided into 2 groups (Gp). Gp1 (12). Wt=286±38 g (m±SD) received Al (dose 200 mg/kg IP) and was subjected to 1/2 hr period of 9.7%O2 followed by 100%O2 GP2 (12). Wt=282±4lg (m±SD) was subjected to the same 2 periods of FiO2 but without prior Al Results: Gp1 With administration of 9.7% O2, Cytaa3 became more reduced, by as much as 100% of total labile signal (TLS), with concomitant fall in BV and HbO and rise in Hb. With administration of 100% O2, Cytaa3 became more oxidized with concomitant rise in BV and HbO and fall in Hb. Gp2. During the hypoxic period (9.7%O2) the changes in Cytaa3, BV, HbO and Hb- were similar to those obtained in Gp1. However on reoxygenation with 100% O2 the Cytaa3 remained reduced despite increases in BV and HbO and fall in Hb-. To evaluate the role of HX and XO in these changes, an additional 6 rats on 100% O2 were infused with I. HX (dose calculated 10 provide a plasma concentration 30 μM/dl). 2.HX with Arabinose (dose 2-4 ml of 1.6 M Sol.) 3. HX + XO (I unit) Infusion of HX, and XO, with or without arabinose, did not result in any change in cytaa3 Our results suggest that allopurinol protect the mitochondrial electron-transport chain from O2− during reoxygenation-reperfusion following cerebral hypoxic-ischemic injury probably by scavenging O2− rather than by inhibition of xanthine oxidase.