Results of maternal 11-hydroxycorticoid (11-OHCS) determination on blood obtained from 4 stages of 44 spontaneous labor cases showed a strong and time-related maternal adrenocortical activation during vaginal delivery. The high 11-OHCS level in the cord blood and the increase of the 11-OHCS concentration in the amniotic fluid suggested the same response of the fetal adrenal. In 14 cases of elective cesarean sections 11-OHCS levels in the cord blood and the amniotic fluid did not rise despite the elevated maternal plasma 11-OHCS content.