Supraventricular tachyarrhythmia has been encountered in 18 fetuses at the Yale-New Haven Medical Center during the past 4 years. Fourteen of these fetuses had supraventricular tachycardia and underwent in utero antiarrhythmic therapy with maternally administered digoxin either alone, or on combination with verapamil, propranolol, or procainamide. Thirteen of the 14 fetuses had successful in utero conversion of cardiac rhythm to normal sinus rhythm. The 14th patient underwent successful therapy after birth. All 14 fetuses survived despite severe fetal hydrops at the time of diagnosis in 13 of 14. The four remaining fetuses had either atrial flutter (3) or fibrillation. Two of the fetuses with atrial flutter died at birth, the 3rd survived after electrical cardioversion at birth. The fetus with atrial fibrillation converted to normal sinus rhythm and survived after maternal administration of digoxin. Using M-mode and pulsed Doppler echocardiography, the nature and electrophysiologic mechanism of the arrhythmia may be deduced. The latter information is reviewed along with the fetomaternal pharmacology of various antiarrhythmic agents to devise a rational antiarrhythmic treatment program.