Blood levels of phenytoin, total thyroxine, and free thyroxine were determined in 10 epileptic women during and following pregnancy. Although total thyroxine concentrations of phenytoin-treated subjects were significantly lower than control values, pregnancy did not appear to diminish the relative increase in total thyroxine seen in normal pregnancy. Free thyroxine concentrations, which were equally depressed at all test times, remained at low euthyroid levels during pregnancy. Following delivery, 3 patients developed free thyroxine levels below 1.0 ng/100 ml. Phenytoin dosage was increased to improve seizure control but produced relatively low blood levels during pregnancy. Following delivery, phenytoin dosage was slightly decreased but produced a significant increase in phenytoin blood levels. The potential effects of phenytoin-related changes in maternal and fetal thyroxine levels are discussed.