We used a sensitive Doppler echocardiographic technique to evaluate in utero the effects of betamethasone on the human ductus arteriosus. Transient, mild constriction of the ductus arteriosus 4-5 hours after the first injection of betamethasone occurred in two of 11 trials. The relatively mild and brief effects on the human ductus of betamethasone in usual doses are probably not clinically significant in most instances. Hence, they do not appear to contraindicate the use of glucocorticoids to promote fetal lung maturation.