This study was undertaken to validate the time domain processing method for measuring (1) the peak velocity in comparison to pulsed-wave spectral Doppler findings in an in vitro system; (2) the volumetric flow in comparison to the actual flow measured by a graduated cylinder in an in vitro circulation; and (3) the volumetric flow in comparison to a transit time flowmeter in a permanently instrumented neonatal lamb model. A prototype implementation of time domain processing in a commercial ultrasound device was used. For velocimetry, both time domain processing and Doppler methods showed low variance, low intrarater variability (0.03 and 0.09%, respectively), high reliability coefficients (97% and 96%, respectively), and a significant correlation (r = 0.96; P < 0.001). For in vitro flow quantification, time domain processing and graduated cylinder methods showed low variance, low intrarater variability (0.09 and 0.01%, respectively), high reliability coefficients (99.60% and 99.96%, respectively), and a significant correlation (r = 0.98, P < 0.001). For in vivo flow quantification, time domain processing and transit time flowmeter showed a significant correlation (r = 0.96; P < 0.001). Within the limits of the in vitro and in vivo experimental conditions, this study proves the validity of the time domain processing sonographic technique for measuring peak flow velocity and volumetric flow.