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The effect on thyroid activity of a systemically ineffective dose of thyroxine (T₄) implanted in the hypothalamus or pituitary of goldfish was tested. Thyroid activity was decreased by T₄ implantation in either location, indicating that T₄ has a negative feedback effect on the pituitary causing a decrease in thyrotrophin secretion, and a positive feedback effect on the hypothalamus stimulating the secretion of thyrotrophin inhibitory factor (TIF). Fish with a T₄ or blank-control implant in the pituitary that had a damaged pituitary stalk, as a result of the operative procedures, were hyperthyroid, suggesting either that TIF is more effective in suppressing thyrotroph activity than T₄ and that the effect of T₄ was masked by the absence of TIF, or, less likely, that T₄ negative feedback in the pituitary is not effective independent of TIF. The results were compared with the information about T₄ feedback in mammals.