Ritodrine hydrochloride infused intravenously at increasing rates of 100 to 300 μg/min inhibited uterine contractility completely in 5 of 6 premature labors. Effective inhibition of the intensity of uterine contractions, which had been unresponsive to other treatments, makes ritodrine a valuable synergetic agent in the combined treatment of premature labor, when single agents fail. A moderate increase in heart rate and pulse pressure suggests an increased cardiac output and placental perfusion during treatment with ritodrine.