The concentrations of hypoxanthine, xanthine, and uric acid in plasma and cerebrospinal fluid (CSF), as well as the urinary output of hypoxanthine and xanthine, were measured in four groups of pigs (three groups with different degrees of hypoxemia and one control group). During hypoxemia with arterial O2tension between 2.1 and 3.0 kPa [group 1, fractional inspired oxygen (FiO2)=0.08], hypoxanthine increased in CSF from a mean basal value of 18.1 to 39.3 µmol/L at death (p < 0.02), in plasma from 25.4 to 103.6 µmol/L (p < 0.05), and in urine from 21.3 to 87.1 nmol/kg/min (p < 0.02). Xanthine changed in a similar way: in CSF from 4.0 to 10.6 /nmol/L (p<0.02), in plasma from 0.7 to 48.1 µmol/L (p<0.02), and in urine from 4.0 to 12.6 nmol/kg/min (p<0.05). Uric acid increased in CSF from 2.7 to 11.6 µmoI/L (p <0.05), and in plasma from 15.4 to 125.0 µmol/L (p<0.02). During hypoxemia with arterial O2tension between 3.0 and 4.0 kPa (group 2, FiO2=0.11), hypoxanthine increased in the CSF from 14.7 to 42.9 µmol/L (p<0.02). Plasma hypoxanthine increased from 20.3 to a maximum of 44.1 µmol/ L (p<0.02), but decreased to initial values by the time of death. The urinary excretion of hypoxanthine increased from 13 to 54 nmol/kg/min (p<0.02). Xanthine increased in the CSF from 3.9 to 13.3µmol/L (p< 0.02), in plasma from 0.6 to 36.6 µmol/L (p <0.02), and in urine from 6 to 25 nmol/kg/min (p < 0.02). Uric acid increased in CSF from 3.1 to 16.3 jumol/L (p< 0.02), and in plasma from 15.3 to 208 µmol/L (p<0.0 0.02). During milder hypoxemia with arterial O2tension between 4.3 and 5.6 kPa (group 3, FiO2=0.14), or in the control group (group 4, FiO2K=0.21), neither of the metabolites changed significantly.