Using a two-dimensional linear array real-time and pulsed Doppler ultrasound system, volume blood flow measurements were made in 86 normal pregnancies, seven cases of fetal bradycardia, and seven cases of fetal tachycardia at the lower thoracic level of the fetal descending aorta. During fetal cardiac arrhythmias, volume blood flow was maintained within the normal range until the heart rates reached around 50 bpm and 230 bpm, after which the volume blood flow diminished. The changes observed suggest that the Frank-Starling mechanism is functional in the fetal myocardium and demonstrate the stability of the fetal circulation.