Neurosympathetic system activity at birth was studied by measuring umbilical arterial plasma catecholamine concentrations in 36 preterm fetuses. Umbilical arterial catecholamine concentrations were correlated with blood gas status, mode of delivery, fetal sex, and fetal heart rate patterns. Significant correlations were observed for plasma norepinephrine and epinephrine versus fetal pH and PO2 and for plasma dopamine versus pH but not PO2. These catecholamine relationships to acidosis and hypoxia were similar to those of our previously published data for term fetuses. Norepinephrine and dopamine concentrations were similar in both preterm and term fetuses; however, epinephrine levels were significantly greater in preterm fetuses than in term fetuses. Increased concentrations of norepinephrine, epinephrine, and dopamine were observed in association with abnormal fetal heart rate patterns. There was no significant effect of fetal sex on the catecholamine secretory response. Similar cord catecholamine concentrations were observed following vaginal and cesarean section delivery, the latter with and without labor. These results indicate that the preterm fetus, like the term fetus, responds to stress at delivery with a graded catecholamine release. The observation of greater epinephrine concentrations in preterm fetuses than in term fetuses may reflect increased secretion or decreased clearance of epinephrine.