Eighty-six fetuses of 21-41 weeks' gestation with arrhythmias were studied with ultrasound and heart rate monitoring. The type of arrhythmia was identified by M-mode studies and was confirmed by postnatal electrocardiogram in 70 infants. The most common arrhythmia was premature atrial contractions (76), followed by premature ventricular contractions (five), paroxysmal supraventricular tachycardia (four), and atrial fibrillation/flutter (one). Doppler echocardiography was performed in 54 fetuses to measure flow velocities across the atrioventricular and semilunar valves. After isolated premature atrial and ventricular contractions, post-extrasystolic potentiation was demonstrated by an increase in fractional shortening (N = 32) of 49 +/- 6% in the right ventricle and 64 +/- 7% in the left ventricle. When post-extrasystolic beats were compared with normal beats, Doppler-determined time-velocity integrals increased 43% across the tricuspid valve, 41% across the mitral valve, 34% across the pulmonary valve, and 38% across the aortic valve. Mean velocity increased significantly after conversion to normal sinus rhythm in the five fetuses with supraventricular tachycardia (P less than .05). By studying the physiologic consequences of fetal arrhythmias using two-dimensional Doppler and M-mode ultrasound, we have documented the presence of post-extrasystolic potentiation after premature contractions, the existence of the Frank-Starling mechanism, and an increase in mean velocity (and therefore in cardiac output) after conversion of fetal tachyarrhythmias to normal sinus rhythm.