The response of the fetal circulation to beta adrenergic stimulation with ritodrine hydrochloride has been investigated by long-term monitoring of the fetal lamb in utero. Ritodrine was infused intravenously either into the ewe or directly into the fetus, and cardiovascular and acid-base responses were measured. Fetal cardiac output and its distribution were measured with the use of radionuclide-labeled microspheres. The output of each ventricle also was measured by means of long-standing implanted electromagnetic flow transducers around the ascending aorta or pulmonary trunk during infusion of ritodrine at various rates into the fetus. Infusion of ritodrine (1.9 mcg. per kilogram per minute) into the ewe caused no change in fetal heart rate, blood pressure, cardiac output, or umbilical blood flow, but did cause an increase in fetal adrenal and myocardial blood flow. Ritodrine infused directly into the fetus produced a marked increase in fetal heart rate and a minimal change in cardiac output. There were no significant changes in fetal or maternal acid-base balance during the ritodrine infusions.