We questioned whether the hypoxia-induced drop in aerobic metabolism in newborns necessitates an oxygen debt, and whether in such a case the debt is payed upon recovery. Aerobic metabolism (VO2 and VCO2) was computed each 30 sec in 15 newborn kittens (age 2-5 days) using an open flow system during normoxia, 30 min of hypoxia, and 30 min of recovery for two levels of hypoxia, 10% and 15% inspired O2. On a separate occasion, venous lactate levels were measured in normoxia and immediately following hypoxia. As expected, VO2 and VCO2 dropped in hypoxia. The decline started within 2 min, and the average decrease during hypoxia was -25% in 15% O2 and -36% in 10% O2. During recovery after 15% and 10% O2 aerobic metabolism was, respectively, below or equal to the normoxic value, whilst lactate was, respectively, the same or above the normoxic value. We conclude that in the kitten (1) the hypoxia-induced drop in aerobic metabolism is accompanied by no (15% O2) or small (10% O2) anaerobic metabolism, and (2) a post-hypoxic metabolic depression can persist during recovery.