ABSTRACT SINCE the rate of thyroid hormone secretion is an important factor in growth rate (Winchester and Davis, 1952) and egg production (Winchester and Scarborough, 1953), studies were undertaken to develop methods for determination of the thyroxine secretion rate in intact fowls. The earlier techniques of simultaneous administration of thyroxine and thiouracil (Mixner et al., 1944) require sacrifice of the fowls and can be used only to measure the average thyroid hormone secretion rate of groups rather than of individual fowls. When I131 became available, Pipes et al. (1950) proposed that the thyroxine secretion rate could be determined in the intact rat by the injection of graded amounts of thyroxine up to the level which would inhibit thyrotropin secretion. This level would be shown by the effect upon the collection of I131 by the thyroid gland and the amount of protein-bound-I131 in the blood. This plan of measuring thyroxine secretion has . . .