Umbilical arterial plasma norepinephrine and epinephrine were measured using a sensitive, specific radioenzymatic assay. Plasma catecholamines were correlated with umbilical arterial blood gases, durations of the first and second stages of labor, duration of rupture of the membranes, fetal heart rate tracings, and fetal sex. Significant correlations were observed for plasma norepinephrine versus fetal pH and PO2 and plasma epinephrine versus pH but not PO2. The majority of the fetal heart rate tracings demonstrated either a normal baseline or mild variable decelerations. The plasma catecholamines and blood gases were similar in these 2 groups. Significant elevations of both plasma catecholamines were observed with those tracings commonly associated with fetal distress; however, the number of infants was small. No sex differences were observed in plasma norepinephrine or epinephrine or in responsiveness. The results suggest that the human fetus at term responds to acidosis and hypoxia with a graded catecholamine release. This may be an important adaptive mechanism.