Studies of the antithyroid action of 1, 1, 3-tricyano-2-amino-l-propene (U-9189) were performed. In rats, this agent induces acute inhibition of the organic binding of iodine, suppresses the formation of thyroxine, and inhibits the conversion of monoiodotyrosine to diiodotyrosine. Following prolonged administration, these actions are supplemented by inhibition of the thyroidal iodide-concentrating mechanism. The antithyroid actions of U-9189 apparently also obtain in man, since this agent is capable of ameliorating human thyrotoxicosis. However, a high incidence of toxic reactions was observed in a small number of patients so treated.