We tested the hypothesis that hypoxic newborn piglets can be successfully resuscitated with lower O2concentrations than 21%. Severely hypoxic, 2-4-d-old, anesthetized piglets were randomly divided into five resuscitation groups: 21% O2(n= 10), 18% O2(n= 9), 15% O2(n= 9), 12% O2(n= 8), all normoventilated, and a hypoventilated 21% O2group (PaCO2; 7.0-8.0 kPa,n= 9). Base excess (BE) reached -20 ± 1 mmol/L at the end of hypoxia. After 3 h of resuscitation, BE had risen to -4 ± 1 mmol/L in the 21% O2, 18% O2, and hypoventilated groups, but was -10 ± 2 mmol/L in the 15% O2group (p< 0.05versus21% O2group) and -22 ± 2 mmol/L in the 12% O2group (p< 0.05versus21% O2group). Four animals died during resuscitation, all allocated to the 12% O2group (p< 0.05versus21% O2group). Somatosensory evoked potentials (SEPs) recovered in 39 of 45 piglets, and remained present during resuscitation in all except the 12% O2group. SEP recovered initially even in six of eight animals in the 12% O2group, but disappeared again in all later during resuscitation. The SEP amplitude recovered to levels not significantly different from the 21% O2group in all groups except the 12% O2group. Plasma hypoxanthine concentrations and extracellular hypoxanthine concentrations in the striatum decreased during resuscitation to levels not significantly different from the 21% O2group in all but the 12% O2group(p< 0.05versus21% O2group). In conclusion, severely hypoxic newborn piglets were resuscitated as efficiently with both hypoventilation and 18% O2as with 21% O2.Abbreviations: BE,base excess;CaO2,arterial O2content;CVSSO2,venous O2content from the sagittal sinus;FIO2,inspired fraction of O2;MABP,mean arterial blood pressure;PaCO2,arterial CO2tension;PaO2,arterial O2tension;SEP,somatosensory evoked potential