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White Leghorn chicks, injected i.p. (days 3 through 40) with 0.5 g/kg or 1.0 g/kg mean body weight of aminotriazole (ATZ), gained less weight than the control animals. In the 1.0 g/kg ATZ group in comparison to the 0.5 g/kg animals, the responses were of greater magnitude and for longer durations of time. In both experimental groups, the thyroid/body weight ratios increased and were greater than those of the controls after day 10. In the higher dosage group, these ratios increased markedly, whereas with the lower dosage group, the ratios stabilized at a lower value. Histological alterations of the thyroid included hypertrophy, hyperplasia and hyperemia. To study the effect of cessation of drug treatment, groups of animals received the same dosages of ATZ through day 20. At the termination of the experiment (day 41), this resulted in a sharp increase in body weight, experimentals weighing as much as or more than the controls. Regarding the decrease in thyroid/body weight ratios, experimentals never attained the low control values. However, upon cessation of drug treatment, a return towards normal thyroid histology was noted. The gross body weight, the thyroid/body weight ratios, the histological analyses of the thyroidal components and the triiodothyronine and thyroxine determinations in the blood serum demonstrate that ATZ is a potent goitrogenic agent which produces a state of functional hypothyroidism.