M-mode echocardiography was performed before and during intravenous beta-adrenergic agonist tocolysis in 13 patients with preterm labor. There was a significant increase in heart rate, fractional shortening, and calculated cardiac output. There was also a significant decrease in both end-systolic and end-diastolic left ventricular dimensions, which resolved in 24 to 36 hours. There were, however, no changes in left atrial dimensions. The data derived from this study suggest that the pulmonary edema occasionally complicating beta-adrenergic agonist tocolysis is not the result of systolic cardiac dysfunction.