Abnormally low uptakes ofiodine-131 were observed in dogs fed commercial diets which contained more than the dogs' daily iodine requirements. To investigate this anomaly three groups of six puppies were fed either (1) a home-pre-pared diet, (2) a commercial diet, or (3) a home-prepared diet supplemented with iodine to the level in the commercial diet. In groups 2 and 3 the uptake of iodine131, and the basal levels of serum thyroxine (T4) and free thyroxine (vr4) were significantly reduced,but the excretion ofiodide in urine and the level of serum thyroid stimulating hormone (TSH) were increased. There was an increased response to the thyroid releasing hormone (TRH)-TSH test in group 2 and to the TRH-T4 test in groups 2 and 3. The results show that the excessive iodine in the commercial diet significantly impaired the puppies' thyroid function and induced hypothyroidism.