Samples of scalp blood were collected from 129 fetuses during the first stage of labor for analysis of plasma norepinephrine and epinephrine concentrations by high-performance liquid chromatography. The catecholamine levels were related to scalp blood pH and fetal heart rate patterns during the 20-minute period preceding the collection of scalp blood. The median norepinephrine level was 9.2 nmol/L (range, 1.3 to 99.7), and the median epinephrine level was 0.5 nmol/L (range, less than 0.5 to 19.2) (n = 111) during the first stage of labor when the pH was above 7.25. The norepinephrine level was considerably higher than that in the resting adult. Significantly higher concentrations of catecholamines were found when scalp blood pH was below 7.26 (p less than 0.001). Maternal analgesia did not influence the fetal catecholamine levels in uncomplicated labor. Significantly higher concentrations of norepinephrine were found during the appearance of abnormal fetal heart rate patterns.