I. On the Thyroid Function of Normal Control RabbitsUsing normal mature rabbits (10 males and 10 females) 2,000-3,000g. in body weight, their serum PBI was measured by a modified distillation method based on BARKER's and CONNOR's methods and their total blood cholesterol by modified BLOOR's method ; 6 male and 6 female rabbits were used for measuring the I131 uptake rate of their thyroid gland following FIELDS's method in principle. The results were in summary as follows : 1. The mean serum PBI content was 4.6±0.71γ/dl (3.7-6.47/dl). No difference was perceptible between the values for males and females.2. The mean total blood cholesterol content registered 82±14.9 mg/dl (59-110 mg/dl).3. The mean I131 uptake rate on the thyroid gland was 21.8±10.4% in one hour value after NaI131 injection ; 29.5±8.8% in 3 hours value 30.5±8.8% in 6 hours value ; 22.0±6.7% in 12 hour value ; 15.7 ± 5.6% in 24 hour value ; 11.4 ± 6.8% in 48 hour value ; 8.5 ± 5.5% in 72 hour value.The mean values for the 6 males were lower than those for the 6 females in the first 24 hours, but in the later hours, the values were higher for the males than for the females.II. The Function of the Thyroid Gland in F1, F4, F5 and F6, Rabbits Descended from Ancestor with Dysfunction in the Thyroid Gland in Every Case for Several Successive Generations1. For successive generation, a breed of rabbits were made to suffer from hyperfunction of the thyroid gland before mating by oral administration of 30 mg/kg of powdered thyroid gland for not less than one month, and the youngs begotten from them (F1, F4, F5, F6) were used for similar measurements as in I above in 50-210 days after birth. It was confirmed that the serum PBI content and the I131 uptake rate of the thyroid rose higher in them than in the normal control, and ever rising higher from generation to generation, so that at last in F6, the mean serum PBI value was heightened to 220% (10.0 ± 1.07 γ/dl) of that of the controls, while the I131 uptake rate rose to 3 folds of the normal in 1 hour value, to 220% in 3 hour and to 190% in 6 hour values. These findings suggest that these offspring rabbits suffer from hyperfunction of the thyroid gland aggravating from generation to generation, so that a complete state of hyperthyroidism is finally brought about in most of the F5 rabbits and later generations.2. In 5 rabbits given powdered thyroid gland, the mean serum PBI value was higher than normal (9.6 ± 1.11γ/dl), the mean total blood cholesterol value a little below normal (78 ± 13.5mg/dl) and the mean I131 uptake rate of the thyroid gland obviously lower and more retarded than in normal cases.3. In the offsprings (F1 and F4) 50-220 days after birth from parents, both male and female, with their thyroid gland ectomized at least one month before mating, in every case for successive generations, the serum PBI value was always subnormal, the value falling of further by every generation, the total blood cholesterol content was above normal and rose higher by every generation, and also the I131 uptake rate was always supernormal and higher in F4 than in F1. From these findings it may be deduced that these offspring rabbits suffer from hypothyroidism mainly consisting in disturbed hormone elaboration in the gland and aggravating from generation to generation.4. In 5 thyroidectomized rabbits, the mean serum PBI value was below normal (3.0±0.44γ/dl), the mean total blood cholesterol content was above normal (118 ± 35.9 mg/dl) and the mean I131 uptake rate in 4 such rabbits was found markedly fallen below normal.