To compare the effect of 21% vs. 100% oxygen during post-asphyxial reoxygenation on brain cell membrane function in the striatum, 20 anesthetized, ventilated newborn piglets were studied: group 1 (normoxia, n = 5), group 2 (asphyxia, no reoxygenation, n = 5), group 3 (asphyxia followed by reoxygenation with 21% O2, n = 5), and group 4 (asphyxia followed by reoxygenation with 100% O2, n = 5). Asphyxia was induced by a stepwise reduction in FiO2 at 20 min intervals from 21% to 14%, 11%, and 8%. Following a total 60 min of asphyxia, piglets in groups 3 and 4 were recovered for 2 h with either 21% or 100% O2. Na+,K(+)-ATPase activity (mumol Pi/mg protein/h) in striatal cell membranes was 31 +/- 1, 22 +/- 2, 32 +/- 2 and 26 +/- 1 in groups 1, 2, 3 and 4, respectively. Na+,K(+)-ATPase activities in groups 2 and 4 were significantly lower than in groups 1 and 3 (p < 0.01). Piglets recovered post-asphyxia for 2 h with 21% O2 had restoration of Na+,K(+)-ATPase activity to baseline levels, while those treated with 100% O2 during recovery had persistent Na+,K(+)-ATPase inhibition of 16%. This could result from increased free radical production during reoxygenation with 100% O2 which could contribute to post-asphyxial cellular injury in the striatum.