Levels of total corticosteroids and unconjugated oestrone and oestradiol-17β in human umbilical arterial and venous plasma at the time of delivery were measured, using competitive protein-binding methods. Increased corticosteroid levels were associated with the establishment of labour and, following delivery at term (38–40 weeks), corticosteroid levels were least when the mother was not in labour and greatest when the mother was in established spontaneous labour at the time of delivery. A surge and peak in corticosteroid levels was observed at 35–37 weeks of gestation in infants delivered by elective Caesarean section and per vaginam following spontaneous labour and thus was not related to labour. Oestrone and oestradiol-17β levels in umbilical venous plasma have also been shown to change with the establishment of labour, oestrone levels increasing and oestradiol-17β levels falling, so that oestrone predominates. Following spontaneous labour with vaginal delivery, there was a significant effect of parity upon oestrogen levels, higher levels of both oestrone and oestradiol-17β occurring in infants from primiparous mothers. This effect of parity was not observed following Caesarean section, either when in early labour or not in labour. It is concluded that fluctuations in corticosteroid and oestrogen levels in the feto-placental circulation are associated with the establishment of labour and may therefore have a regulatory role in human parturition.