Neonatal mice transgenic (tg) for copper/zinc-superoxide dismutase (SOD 1) were found to have greater brain injury after hypoxia-ischemia (HI) than their nontransgenic (ntg) littermates (Pediatr Res 39: 204, 1996). No upregulation of catalase activity was found in neonatal SOD tg mice versus their ntg littermates, before or after HI (Soc Neurosci Abst 22: 1430, 1996). To further assess the roles of anti-oxidant enzymes after HI, we measured glutathione peroxidase (GP) activity and hydrogen peroxide (H2O2) concentration in the brains (cortex and hippocampus) of neonatal SOD tg and ntg mice before and after HI. 15 litters (164 pups) of CD1 mice tg for SOD were identified by gel electrophoresis. On postnatal day 7, half of the pups were subjected to unilateral carotid ligation and 30 mintues of hypoxia in 8% O2. GP activity and [H2O2] were assessed in control mice (no HI), immediately after HI, and 24 hours after HI. Contrary to catalase, GP activity dropped precipitously after injury, in both SOD tg and ntg mice: from 129.4 ± 47.3 U/(mg protein) in SOD tg cortex before injury, to 15.9 ± 7.4 U/(mg protein) in SOD tg cortex 24 hours after injury (p<0.01; similar findings observed in the hippocampus and in ntg brains). Although there was a trend towards greater GP activity in ntg vs. SOD tg brains, it was not significant. Aminotriazole inhibition of catalase activity (a qualitative measure of [H2O2]), was similar between tg and ntg brains before and 2 hours after injury. However, 24 hours after injury, greater aminotriazole inhibition was observed in the tg mice vs. their ntg littermates (p<0.01), indicating greater [H2O2] in these brains. Taken together, these findings suggest that the fall in GP activity after perinatal HI leads to an accumulation of H2O2 in SOD tg brains. This could explain the increased injury after HI previously observed in these tg mice, and suggests a role for GP in the mechanism of oxidative damage after HI in the neonate. This research was supported in part by K12 NS01692-04, RO1 NS33997-01A1, NS35902, NS32553, and AG08938.