Hypoxemia was induced in pigs by artificial ventilation with 7% oxygen. Death occurred after 21--99 min (mean 48 min). The plasma hypoxanthine and base deficit concentrations increased linearly during hypoxemia and were significantly elevated after 5 min compared to initial values. The correlation between hypoxanthine and base deficit was good (p < 0.001, r = 0.68). The rate of increase of plasma hypoxanthine concentration was correlated with survival time and a significant correlation was found between the negative slope of hypoxanthine increased and survival. Death occurred in all animals when the hypoxanthine concentrations exceeded about 125 mumol/l. These findings indicate that the plasma concentration and the rate of increase of hypoxanthine specifically and sensitively reflect tissue hypoxia.