The minimum iodine requirement of the rat was determined by feeding various groups of rats on a goitrogenic diet to which different amounts of iodine (as KI) were added. Using as a criterion the smallest amount of iodine necessary to prevent any significant thyroid enlargement, the minimum iodine requirement was found to be approximately 1–2γ per rat per day. This intake of iodine yielded a concentration of 0.11–0.18 per cent iodine (dry basis) in the thyroid. The daily intake of iodine necessary to yield a concentration of 0.1 per cent iodine (dry basis) in the thyroid was found to be approximately 0.9 γ. The inclusion of 4 per cent Osborne-Mendel salt mixture in synthetic diets for experiments with rats provides from 5–10 times the minimum iodine requirement of the rat. Using for comparison the results obtained in the rat, the minimum iodine requirement of the human was calculated to be approximately 60–120 γ per day.