The effects of antiepileptic drugs on thyrotropin (TSH), thyroid hormone, and growth hormone (GH) concentrations in sera of 16 pregnant women with epilepsy were studied. The drugs markedly depressed thyroxine (T4) concentration in maternal and umbilical cord sera, and partly depressed TSH in maternal and umbilical artery sera and GH in umbilical artery sera, but not triiodothyronine (T3) or reverse T3 (r-T3) concentration. The effect of antiepileptic drugs on T4 concentration in maternal serum was not influenced by pregnancy. Ratios of hormone levels in maternal serum compared with those in umbilical cord serum revealed a pattern similar to that seen in normal pregnant women. They suggest that transplacental passage of TSH, thyroid hormone, and GH are not markedly affected by antiepileptic drugs.