The use of beta-adrenergic agonists in high-risk pregnancies has shown evidence of favorable effects on the fetus. Intravenous injections of133Xe were given to evaluate the effects of short-term administration of fenoterol (3μg/min) and isoxsuprine (150 jug/ min) on the intervillous and myometrial blood flow in a series of 48 women during the last trimester of pregnancy. Both fenoterol and isoxsuprine treatment increased the maternal heart rate significantly. There was a significant rise in myometrial blood flow when fenoterol was given, but the intervillous blood flow did not change significantly during the administration of either isoxsuprine or fenoterol. Previous oral isoxsuprine treatment did not diminish the cardiac effect of intravenous fenoterol, but the improvement in myometrial blood flow was eliminated. This result indicates that β-adrenergic agonists may have a specific dilatational effect on the myometrial blood vessels. From the heniodynamic point of view, the β-adrenergic a onists have a limited value in the treatment of chronic fetal asphyxia or intrauterine fetal growth retardation.