The hypothesis tested was whether marginal iodine deficiency for a period of 6 wk affects iodothyronine deiodinase activities in liver and brain of rats. Male rats were fed purified diets either defident or sufficient in iodine; the diets were fed on a restricted basis (60% of ad libitum intake). Body weight gain of the two groups was comparable, Iodine deficiency was evidenced by increased thyroid weight (26%), reduced urinary iodine excretion (80%), and reduced plasma T4 concentrations (22%). Activities of liver type I and brain type III deiodinase were unchanged, but the activity of type I] deiodinase in brain was increased (28%) in the iodine-deficient rats. Food restriction per se significantly lowered T 3 (30%) and I"4 (22%) concentrations in plasma and decreased type III deiodinase activity in brain (30%). These results indicate that in marginal iodine deficiency the activities of hepatic type I deiodinase and brain type III deiodinase are unchanged, whereas that of brain type II deiodinase is increased.Index Entries: Rats; iodine deficiency; food restriction; thyroid; deiodination; liver; brain.*Author to whom all correspondence and reprint requests should be addressed. Biological Trace Element Research 237VoL 40. I ~94