A new method of assaying thyrotropic hormone is described. The method depends on labeling the I compounds in the thyroid of a guinea pig with I131, and the inhibition of the secretion of thyrotropin by the animal'' s own pituitary by giving adequate amounts of thyroxin. In animals so treated, the rise in the blood radioactivity following an injection of thyrotropin, is a measure of the amount of thyrotropin injected. Sensitivity of the method is such that good responses are obtained with 10 [mu]g of U.S.P. Thyrotropin Reference Substance, given intravenously; the limit of detection is of the order of 2 [mu]g. Accuracy of the method is such that when preparations were compared by the intravenous route, using 16 animals, the standard error of the mean ratio of the activities was +5%, -4%. Thyrotropin content of blood samples, taken from animals and people in various conditions, is reported.