Oxygen consumption and phosphorylation of beef thyroid slices were determined in vitro in the presence of thiocyanate at concentrations ranging from 10-2M to 10-7M. Iodide uptake and its organic binding were also measured under similar experimental conditions. Thiocyanate increased oxygen consumption and decreased phosphorylation and rate of oxidative phosphorylation of the thyroid at concentrations above 10-4M. In the same concentration range, thiocvanate depressed the uptake of I131 by the thyroid. From the evidence presented, it was inferred that thiocyanate exerted its action of depressing iodide concentration of the thyroid through energetical mechanism, that is, through depression of much, if not all, of the energy supplied by highenergy phosphate bonds for the active transfer and maintenance of concentration gradient of inorganic iodide by the thyroid tissue.