A study of iodide transport in the iodine-iodide-black lipid membrane system was carried out. Conductance data suggests that the transport process is first order with respect to both the iodine and iodide concentrations. To account for this and other experimental observations, a mechanism of iodide transport is postulated in which the formation of I is the rate determining step. The antithyroid agents, potassium thiocyanate, 6-n-propyl-2thiouracil, and 2-L-thiolhistidine were observed to inhibit iodide transport. It is thought that potassium thiocyanate acts in competition with iodide for iodine complex formation. The two other thio-compounds, being much more effective inhibitors, are believed to inhibit transport by reducing the iodine in the system. Perchlorate, also a well known goitrogen, was observed to have no effect on iodide transport. It is postulated that it affects thyroidal iodide metabolism in steps subsequent to iodide transport, perhaps by interaction with iodide peroxidase. In addition, histidine was shown to affect transport in a manner similar to thiocyanate. As a result, it is suggested that histidine may form a complex with iodine. Bromide, which has no antithyroid effect, was observed to be transported by iodine almost as easily as iodide. The proposed mechanism of iodide transport is used to explain the mode of action of the antithyroid agents on thyroidal iodide transport and is shown to account for many of the experimental and clinical observations in the literature. Further experiments are suggested to substantiate the proposed mechanism.