The progesterone concentration in the umbilical venous and arterial plasma has been measured by radioimmunoassay. In 35 normal spontaneous vaginal deliveries the concentration in the umbilical venous plasma was highly correlated with the difference between venous and arterial plasma concentrations. There was no correlation of progesterone levels with infant birth weight or placental weight, but slightly higher amounts were found in samples taken from women with female fetuses than those with male fetuses. There was also a significant correlation of venous progesterone concentration with duration of labour. In 14 subjects where forceps were used to assist delivery because of prolonged second stage labour, progesterone levels were found to be elevated, whereas in nine cases of elective Caesarean section they were reduced. It is suggested that the fetus may stimulate progesterone secretion by the placenta in response to stress and that there may also be a fetal sex-related response.