Many mechanism may participate in the inhibition of thyroid function by excess of iodine. The present study was aimed at answering the question, whether the excess of iodine influences the activity of type I thyroxine 5'-deiodinase. To 40 female rats fed with standard diet a dose of 42 micrograms of iodine daily was administered into the stomach through the gastric tube. Control group consisting of 10 rats received standard diet only. Part of the animals was sacrificed after each of the consecutive time intervals (2, 4, 7 and 14 days of treatment with iodine) and the activity of deiodinase in the thyroid as well as the blood serum levels of hormones, T3 and T4, were determined. A decrease in the activity of deiodinase in comparison with control group was observed during the whole experiment, the highest inhibition occurring after 4 days of treatment with iodine. A fall in the concentration of T3 was observed after the second and fourth dose of iodine, with no change in T4 concentration. The results of the experiments indicate that the excess of iodine inhibits the activity of type I thyroxine 5'-deiodinase in the thyroid.