The circulatory consequences of cardiac arrhythmia and its compensatory mechanisms were examined in utero in 37 fetuses. A combined real-time and 2-MHz pulsed Doppler technique was used to measure blood velocity for waveform analysis and flow estimation in the descending thoracic aorta of the fetuses. The pattern of blood velocity in the inferior vena cava was studied to classify the arrhythmia. Despite severe cardiac arrhythmias the aortic blood flow was within normal limits in all but one fetus which had associated cardiac malformation and congestive failure. The rising slope and the peak value of the maximum aortic velocity were significantly increased in the postpremature beats and in fetuses with atrioventricular block. The results support the validity of Frank-Starling's law for the fetal myocardium.