Chorion was obtained from 10 healthy pregnant women at term gestation who were delivered either vaginally (n = 5) or by elective repeat caesarean section before the onset of labour (n = 5). Epidermal growth factor (EGF) receptors in chorionic tissue were evaluated by means of freeze fracture technique (label fracture). They were found to be significantly less, but more frequently coupled, in specimens obtained after labour than before the onset of labour. These findings suggest that a functional activation of EGF receptors in human chorion might occur during labour.