Comparisons of the effects of thiouracil treatment, surgical thyroidectomy, and radiothyroidectomy on body and organ weights of White Leghorn chicks were made to determine the most effective method of producing hypothyroidism. Radiothyroidectomy was deemed most effective and in a second experiment radiothyroidectomy, castration, and replacement of thyroid hormone and/or androgen were utilized in setting up the following groups of male chicks: (1) Control, (2) Castrates, (3) Thyroidectomized, (4) Thyroidectomized castrates, (5) both procedures plus partial thyroid hormone replacement, (6) both procedures plus exogenous methyl testosterone, (7) both procedures plus androgen and thyroid. Castration was done when the birds were about 1-week-old. Radiothyroidectomies were done on Day 11. Hormones were mixed with feed. Comb size and body weight were measured weekly, and daily food consumption was measured throughout. The experiment was terminated when the birds were 5 weeks old, and the following conclusions were drawn from the measurements made: (1) Castration had no effect on body weight in the presence or absence of thyroid hormone, whereas thyroid lack decidedly reduced body weight gain. (2) Exogenous androgen reduced food intake and body growth in Group 6, but not when endogenous or exogenous thyroid was present. (3) Both androgen and thyroid hormone are involved in comb growth, but either alone resulted in more growth than occurred in the thyroidectomized castrates. (4) Some androgen is produced by the significantly smaller testes of the thyroidectomized chick. (5) Lack of thyroid hormone (Groups 3, 4, and 6) resulted in larger relative liver weights and greatly increased liver glycogen. (6) Subnormal exogenous thyroid hormone (Group 5) gave less than normal body and comb growth, but allowed normal liver size and glycogen content to be maintained. (7) Thyroid weight was significantly reduced by castration. (8) The highest comb water content was in those groups having both thyroid and androgen. (9) Adrenal weight was unaffected by any of the treatments.