Antepartum betamethasone treatment in pregnancies which terminated at less than 37 weeks of gestation resulted in a rapid and significant decrease in cord plasma cortisol levels. Mean values +/- S.E. were 5.17 +/- 0.39 microng per 100 ml. in 90 untreated cases, 2.17 +/- 0.32 microng per 100 ml. in 29 cases treated at least 24 hours prior to delivery, and 2.27 +/- 0.38 microng per 100 ml. in 18 cases in which treatment was begun within 24 hours of delivery. The mean cortisol concentration in five amniotic fluid samples obtained after betamethasone administration (0.93 +/- 0.19 microng per 100 ml.) was significantly less than in seven control samples (1.85 +/- 0.21 microng per 100 ml.) These results suggest that betamethasone suppressed endogenous cortisol production by maternal and/or fetal adrenals. However, blood cortisol levels during the first days of life of treated neonates were considerably increased in comparison to those in cord plasma, suggesting that there was rapid recovery of their adrenal function.