Iodine status of aborted fetuses (11-25 wk gestation) was determined on the basis of maternal thyroid status and urinary iodine excretion. Fetal cerebral cortex thyroxin and 3,5,3'-triiodothyronine (T3) contents peaked at 15-18 wk gestation and then fell in iodine sufficiency. In mild iodine deficiency T3 concentration was maintained at a higher level until week 22, although it were still significantly less than in the iodine-sufficient group. Reverse T3 (rT3) rose from 11 to 22 wk gestation without any effect of iodine status. The activity of 5' and 5-monodeiodinase also increased from 11 to 22 wk gestation, with 5' monodeiodinase activity significantly increased by mild and moderate iodine deficiency. In contrast, cerebral cortex 5-monodeiodinase activity was significantly lower in moderate iodine deficiency at 15-18 wk gestation. The observations indicate that cerebral cortex 5' and 5 monodeiodinase activities are modulated in iodine deficiency to enhance T3 production from thyroxin (T4) during the period that coincides with neuroblastogenesis.