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Soybean derivatives were tested in rat through acute experiments of 3 to 24 hours and two semichronic experiments of 16 and 29 days. The acute essay were realized with Total Extracts (TEs) obtained from Defated Soybean Flour (DSF) by precipitation in an aquous medium (pH4,2) and posteriorly in acetone (2 vol.). It was observed that the Total Autoclaved Extract (TAE) administered by gastric tube after 6 and 24 hours decreased the percentage of iodine (^13lI) uptake by 100 gr. of rat. The Total Extract, without previous autoclaving showed effect on the gland after 6 hours and lost its activity 24 hours after its administration. TEs obtained from Comercial Soybean Products as: Proteic Concentrate, Tosted Flour and Milk also provoked a decrease in percentage of iodine (¹³¹I) uptake after 24 hours by 100 gr. of rat. The semichronic experiments were realized with soybean fraction products, which were incorporated to experimental diet. The first semichronic essay of 16 days, showed a reduction in percentage of iodide (¹³¹I) uptake by 10 mg of thyroid and an increase of the triiodothyronine-binding capacity of rat serum. In the second semichronic of 29 days, we had an increase in the percentage of iodine (^13lI) uptake by 10 mg of thyroid caused by the factor in study and no alteration of seric hormones. We also assayed the thyroid hormones and their precursors in this essay and observed na increase of monoiodotyrosine (MIT), triiodothyronine (T₃) and thyroxine (T₄) and a decrease of diiodotyrosine (DIT) and inorganic iodine. We also observed, and increase in the MIT/DIT ratio and decrease in T₃/T₄ ratio. In preliminary physicochemical tests, the fraction sephadex G-25 showed a positive reaction for ninhidrin, Molish and flavonoids.