A 3.0 MHz pulsed Doppler echocardiography was used to estimate instantaneous stroke volume (SV) and cardiac output (Q) in eight men during steady-state supine (S) and upright (U) exercise at 300 kpm/min. The mean transients in heart rate (HR), SV, and Q for the first 20 s of exercise in each posture were then determined. Center-line blood velocities were obtained in the ascending aorta with the transducer positioned manually in the suprasternal notch. Mean supine values for SV and Q at rest and exercise were 111 ml and 6.4 1/min and 112 ml and 9.71/min, respectively. The corresponding results for U were 76 ml and 5.61/min and 92 ml and 8.41/min, respectively. These values compare favorably with previous studies utilizing invasive procedures. The transient response of Q following the onset of exercise in U was about twice as fast as in S because of the rapid and almost immediate upsurge in SV. In S, only HR served to augment Q, as SV initially fell. The faster rise in aortic flow in U with exercise represented an additional volume (184 ml) of blood passing through the aorta compared with S in the first 20s. This must be related to the rapid mobilization of pooled venous blood from the leg veins during U.