Extract: The hypoxanthine concentration in plasma was found to be a sensitive parameter of hypoxia of the fetus and the newborn infant. The plasma level of hypoxanthine in the umbilical cord in 29 newborn infants with normal delivery varied between 0 and 11.0 μmol/liter with a mean of 5.8 μmol/liter, SD 3.0 μmol/liter. Compared with this reference group the hypoxanthine concentration in plasma of the umbilical cord in 10 newborn infants with clinical signs of intrauterine hypoxia during labor was found to be significantly higher, with a range of 11.0—61.5 μnmol/liter, with a mean of 25.0 mu;umol/liter, SD 18.0 μmol/liter. The plasma level of hypoxanthine in two premature babies developing an idiopatic respiratory distress syndrome was monitored. The metabolite was found to be considerably increased, in one of them more than 24 hr after a period of hypoxia necessitating artificial ventilation. The hypoxanthine level in plasma of umbilical arterial blood was followed about 2 hr postpartum in three newborn infants with clinical signs of intrauterine hypoxia. The decrease of the plasma concentration of the metabolite seemed to be with a constant velocity, as it was about 10 μmol/liter/hr in these cases. A new method was used for the determination of hypoxanthine in plasma, based on the principle that decreased when hypoxanthine is oxidized to uric acid. Speculation: These preliminary results indicate that the hypoxanthine concentration in plasma is a sensitive parameter of hypoxia. It is expected that this metabolite will express the degree of hypoxia quantitatively and regardless of the etiology of the hypoxia. The excretion of hypoxanthine and other purine metabolites in urine might also have diagnostic value.