Two cases of fetal tachyarrhythmias were treated in utero. One case showed supraventricular tachycardia which was successfully treated with procainamide. The other case was an atrial flutter with 2:1 atrioventricular conduction (AVC) which was treated with digoxin and verapamil. In both cases, the blood flow in the descending aorta was observed using pulsed Doppler ultrasound during therapy. In the first case, the interval of blood flow peaks was suddenly prolonged to normal heart rate, and in the second case, the blood flow peaks appeared irregularly which indicated that (AVC) was intermittently blocked. The blood flow pattern represented the mode of cardioversion. The blood flow velocity was also measured in both cases; it decreased transiently during therapy representing a depression in cardiac function. The cause of this was thought to be the negative inotropic effects of procainamide and verapamil. In spite of these negative inotropic effects, a greater R-R interval means a greater stroke volume. This relationship has a favorable effect on the fetal circulation as a result of therapy for fetal tachyarrhythmias.