Ritodrine (trade name Yutopar) is beta-2 adrenergic agonist used to stop premature labor. Ritodrine binds to beta-2 adrenergic receptors on the outer membrane of the myometrial cell, activates adenyl cyclase to increase the level of cAMP which decreases intracellular calcium and leads to a decrease of uterine contractions. In addition to stimulating the beta-2–adrenergic receptors of the uterine smooth muscle, ritodrine stimulates beta-adrenergic receptors of bronchial and vascular smooth muscles. The cardiostimulatory effects, including increased cardiac output, increased maternal and fetal heart rates, and widening of the maternal pulse pressure, are probably due to relaxation of the vascular smooth muscle. Relaxation of vascular smooth muscle stimulates the beta-1–adrenergic receptors and the reflex response to blood pressure. Also, during intravenous administration, ritodrine transiently increases maternal and fetal blood glucose and maternal plasma insulin concentrations. Other metabolic changes include increased cAMP, lactic acid, and free fatty acids, and decreased serum potassium concentration. Most side effects of β2 agonists result from their concurrent β1 activity and include the increase in heart rate, rise in systolic pressure, decrease in diastolic pressure, chest pain secondary to myocardial infarction, and arrhythmia. Beta-agonists may also cause fluid retention secondary to decrease in water clearance, which when added to the tachycardia and increased myocardial work, may result in heart failure. In addition, they increase gluconeogenesis in the liver and muscle resulting in hyperglycemia, which increases insulin requirements in diabetic patients. The passage of β agonists through the placenta does occur and may be responsible for fetal tachycardia, as well as hypoglycemia or hyperglycemia at birth.