In 18 healthy neonates, cardiac output was measured by range-gated Doppler echocardiography with simultaneous two-dimensional echocardiography, using the apical approach. Maximal and modal flow velocities were compared by using spectral analysis of the Doppler signal. Blood flow velocities were measured in the left ventricular outflow tract and in the ascending aorta. Cardiac index (cardiac output per kg body weight), with the sample volume placed in the left ventricular outflow, averaged 170 +/- 45 ml/min/kg in the maximal flow velocity mode and 133 +/- 39 ml/min/kg in the modal mode, respectively (p less than 0.05). Sampling in the ascending aorta gave cardiac index values of 198 +/- 73 ml/min/kg in the maximal velocity and 167 +/- 58 ml/min/kg in the modal velocity mode. Maximal flow velocities in the ascending aorta came closest to published data obtained by invasive and noninvasive methods.