The effect of terbutaline sulfate on left ventricular size and performance was studied by M-mode echocardiography in pregnant women with premature labor. Patients with uterine activity initiated during either oxytocin challenge testing or induction of labor served as a comparison group. During terbutaline therapy, heart rate, ejection fraction, and cardiac output increased significantly. End-diastolic volume and systolic blood pressure (BP) were unchanged, and diastolic BP and end-systolic volume fell. No changes in echocardiographic or hemodynamic parameters were present during oxytocin-induced uterine activity. Terbutaline, as currently used to prevent premature labor, is a potent inotropic and chronotropic agent. Pulmonary edema accompanying terbutaline treatment is probably not due to cardiac failure.