Thyroid-stimulating hormone was measured in fetal blood samples obtained by cordocentesis before and after blood transfusion in Rh-affected pregnancies at 25 to 37 weeks' gestation. In eight of 26 study subjects thyrotropin-releasing hormone was given to the mothers immediately after the pretransfusion samples were obtained. In this group the fetal thyroid-stimulating hormone concentrations in the posttransfusion samples were significantly higher than those before the transfusion.