In cases of non-immune hydrops fetalis caused by tachyarrhythmias, the transplacental passage of antiarrhythmic drugs may be hampered. When this is proven by fetal blood sampling in cases of tachyarrhythmia refractory to transplacental treatment, additional administration of antiarrhythmic drugs into the fetus is necessary and seems to improve the results. Although injections of antiarrhythmic agents in fetal ascites are also highly effective, intravascular administration by sonographic guidance is to be preferred. Then, simultaneous measurements of fetal and maternal drug levels are possible for the evaluation of pharmacokinetics and for monitoring the antiarrhythmic therapy.