The effectiveness of radiothyroidectomy and of the feeding of thiouracil, propylthiouracil, and methimazole in producing hypothyroid changes in young cockerels was studied. Changes in glycogen content and relative size of the liver are emphasized as sensitive indicators of severe hypothyroidism. Changes in body, comb, testes, thyroid, and bursa weights are also considered. It was concluded that: (1) 0.1, 0.15, and 0.2% thiouracil (TU) are equally effective in producing hypothyroid changes up to age 3 weeks. (2) An equimolar amount of propylthiouracil (PTU) is slightly more effective than 0.2% TU up to age 30 days. (3) Radiothyroidectomy (RT x) and 0.2% PTU are equally effective to age 5 weeks, but PTU has a greater depressive effect on body weight because of its probably disagreeable taste and consequent effect upon food intake. (4) An amount of methimazole at 0.1% was as effective as 0.2% or 0.3% TU to age 5 weeks, but 0.05% and 0.025% methimazole were relatively ineffective except in thyroid weight changes. (5) Amounts of iodinated casein one-third to one-fourth the amount equivalent to normal thyroid hormone secretion when added to TU diets allowed normal growth in most organs measured, and normal liver glycogen, but the thyroid hypertrophied. Further reduction of iodinated casein to one-fifteenth to one-twentieth the normal thyroid hormone equivalent resulted in moderate liver hypertrophy and above normal glycogen deposition. (6) The twice daily injection of PTU produced liver changes characteristic of TU feeding. (7) The effects of TU on liver change persisted from between 27 and 50 hours after the withdrawal of TU from the feed. (8) Onset of liver changes after initiation of TU feeding required 2–4 days depending on age, and occurred earlier than changes in thyroid weight. (9) Interruption of a TU feeding regime by an overnight fast, although lowering liver glycogen markedly, did not obviate the TU effect during refeeding. (10) Withdrawal of normalizing exogenous thyroxin from RT x chicks resulted in liver hypertrophy and increased glycogen accumulation within 48 hours. Comparative responses of liver in rat and chick are discussed.