23 gland or released to the body where they influence growth, development, and metabolism, has been understood for some time (Salter, 1940). Hence, when radiofodine became available in the late 1930s (Livingood and Seaborg, 1938), it found prompt utility among thyrofdologists and physicians who used the iodine isotope to trace pathways of the element in the body first in rabbits (Hertz et at., 1938) and then in rats (Perlman et al., 1941). Much of the early use was related to diagnostic evaluation of patients with thyroid disease (Hamilton and Soley, 1940; Hertz et al., 1942; Hertz and Roberts, 1942a; Hamilton et al., 1943; Quimby and McCune, 1947; Soley and Miller, 1948; Werner et al., 1948; and Skanse, 1949). The concentration of radioiodine by the thyroid in developing young was also studied. Thyroid glands were shown to take up !31I in utero in the rat (Gortman and Evans, 1941), the mouse (Speert et al., 1951), and the hanster (Kansborough and Seay, 1951), and prior to hatching in the chick (Hansborough and Kahn, 1951). The human fetal thyroid also was shown to concentrate radio- fodine early in tts development (Chapman et al., 19483). Not only was the transfer of radioiodine across the placenta recognized, but so, too, was its movement across the mammary gland, as evidenced in studies on lactating and suckling mice (Rugh, 1951). After these early reports, considerable informat ton continued to be produced on the metabo)ise of 1311, primarily because of the use of the radionuclide ifn medicine and in phys fol ogical and endocrinological studies. The widespread use of the radiofodine uptake test tn diagnostic medicine resulted in increase understanding of how |31J was handled by the thyroid and the hody. Thyroid uptakes of human neonates (2-3 days of age) were first dane ir the mid-1950s (Van Kiddlesworth, 1954b). Newborn infants were found to be myer thyrold-like, with regard to thetr 13!] uptakes, when ranparet tn atults,