Plasma hypoxanthine (HX) concentrations were measured in 16 acutely exteriorized fetal lambs during induced, graded hypoxia, and the values found were compared to established indicators of poor oxygenation in the fetus, such as oxygen saturation, blood gases, pH, base excess, and lactate. Highly significant correlations were found to all parameters of hypoxia except PaO2. When the duration of hypoxia was taken into account, the HX concentrations showed highly significant correlation to constructed variables of oxygen debt. In multiple correlation analysis, pH turned out as the strongest single variable, explaining 80% of the variance in HX concentration with the remaining parameters increasing the value to 87%. The highest concentrations of HX were obtained when hypoxemia was combined with acidemia or with a large fetomaternal PaCO2-gradient which correlated to a reduced cardiac output in the fetus. These findings appear to corroborate the conclusion that plasma HX concentration reflects tissue hypoxia in the fetal lamb.