A 5 MHz range-gated portable pulsed Doppler velocity meter was used to measure mean ascending aortic blood flow velocity noninvasively. Studies were performed from a suprasternal approach in 18 preterm infants with patent ductus arteriosus. Measurements were made in each patient before and after medical or surgical closure of the PDA. The internal ascending aortic diameter was determined echocardiographically and aortic cross-sectional area calculated according to the equation AAo = pi d2/4. Ascending aortic blood flow was computed as QAo (ml/min) = VAo (cm/sec) x AAo (cm2) x 60 (sec/min). Prior to PDA closure, QAo averaged 343 ml/min/kg, well above predicted normal values. After PDA closure, QAo fell to 252 ml/min/kg, significantly lower than the preclosure level (P less than 0.001), but slightly higher than the mean cardiac output of healthy newborn infants. The mean QAo after surgical ligation of the PDA was closer to the predicted normal value than after treatment with indomethacin alone. This study reflects the effect of left-to-right ductus shunting on left ventricular output and emphasizes the demands placed on the neonatal left ventricle by PDA.