Determinations of total iodine content, levels of thyroxine (T4) and triiodothyronine T3), and protein concentrations were made on amniotic fluid. A total of 218 samples obtained at various stages of pregnancy, from both normal and pathological pregnancies, were studied. Normal values for the iodine and hormone concentrations are presented. Several amniotic fluid samples showed greatly elevated iodine levels, thought to be the result of maternal iodine intake. This study demonstrated that such elevated levels can be produced by urography with an iodinated medium, by thyroid extract therapy, and by vaginal therapy with an iodinated agent. Levels of T4 and T3 in amniotic fluid decreased slightly from the second to the third trimester' and were unrelated to the total iodine levels. Despite reports in the literature of fetal hypothyroidism produced by amniography with a fat-soluble medium, no thyroid defect was observed in this series in the presence of elevated amniotic fluid iodine levels or when a water-soluble iodinated medium was used for amniography.