The effect of epinephrine (E) and 5-hydroxytryptamine (5-HT) on bovine thyroid peroxidase activity present in the 30,000 xg subcellular fraction and on the solubilized form of the enzyme was measured. 10^-3M and 10^-5M E and 5-HT inhibited the activity of the particulate peroxidase as well as the solubilized enzyme, measured by tyrosine iodination technique. The solubilized form of the enzyme was also inhibited as evaluated by guaiacol and triiodide assays. Other amines such as norepinephrine and tyramine also inhibited thyroid peroxidase activity. This inhibitory effect of biogenic amines on thyroid peroxidase activity which is opposite to the effect they have on thyroid iodine organification, may be present even in those conditions where the stimulatory effect of the amines is observed, or only under particular circumstances such as those in which hydrogen peroxide is not the limiting factor in the iodination process.