The concentration of ACTH in maternal antecubital venous plasma before, during and after labor and in cord plasma of newborn infants at the time of delivery were measured by radioimmunoassay, and analyzed statistically. In 7 healthy women in the 10th month of pregnancy the concentrations of plasma ACTH just before delivery were statistically higher than the levels found before the onset of labor (p < 0.001). The fetal plasma ACTH concentration was much higher than the maternal level. The difference between the maternal and fetal ACTH concentrations was statistically significant (p < 0.005). No ACTH was detected in the cord plasma of two anencephalic infants. Moreover, after ACTH infusion into 4 healthy women during labor the concentrations of ACTH in their plasma increased to about 2.2 to 10.4 times the control values, while the concentrations in the cord plasma of their newborn infants decreaseed significantly. The difference between the ACTH concentrations in infants delivered from normal women and from women infused with ACTH was significant (p < 0.01). It is concluded from these results that ACTH does not cross the placental barrier.