We evaluated levels of corticotropin-releasing factor in the plasma and amniotic fluid of women who had spontaneous vaginal delivery or elective cesarean. Corticotropin-releasing factor binding sites were also studied in placental tissue collected from vaginal or cesarean birth. Plasma samples were collected hourly from seven women from the onset of labor until delivery, and from ten women before and during elective cesarean. Amniotic fluid samples were collected from 40 women at different stages of labor and from ten women during elective cesarean. Maternal plasma corticotropin- releasing factor levels increased during labor, showing the highest values at delivery. No significant differences in amniotic fluid immunoreactive corticotropinreleasing factor levels were observed at the different stages of cervical dilatation. At cesarean, maternal plasma levels did not differ significantly from those found before surgery, and in the amniotic fluid they were similar to those found in pregnancy. The number of125I-corticotropin-releasing factor binding sites in placental tissue was higher after vaginal than after cesarean delivery. These results suggest that corticotropin-releasing factor secretion is activated by the stress of labor.