Spectral values of iodinated compounds in the thyroid were followed by means of I131 and I127 tests in rats and guinea pigs. The rate of biogenesis of thyroid hormones was found to be slowed down in guinea pigs so that 12 hr after I131 administration there was a predominance of MIT over DIT, while 2 minutes after I131 administration to rats DIT predominated over MIT. The following ratio of molecular representation was found chemically (I127): I’:T3:T4:MIT:DIT 1.00:1.20:2.42:7.61:10.44 in rats and 1.00: 1.39:3.70:6.11:6.28 in guinea pigs. In the chemical (I127) analysis, an unidentified compound (F) appeared at the front of the chromatograms, representing 4.04% in rats and 11.77 % in guinea pigs. In a further observation, the chemical analysis yielded a larger amount of T4, T3 and substance F than of MIT and DIT, while the I131 test showed only the presence of I’, MIT and DIT following the administration of MTU. These results led to the expression of a hypothesis on the existence of a heterogeneity and of a 2-compartment theory in the thyroid. It seems that a certain measure of caution, and a greater reserve than was often the case in the past, will have to enter into the interpretation of results derived from I131 tests in thyroid studies. This is all the more necessary since reports appear with increasing frequency tending to show that there exists in the thyroid not only a morphologic and histochemical but also a functional heterogeneity. Triantaphyllidis (1) found a significantly higher specific activity in the substances secreted from the thyroid within the first 4 days following I131 administration than after a longer interval. Her concepts of a functional heterogeneity have been supplemented by further studies (2). Discrepancies between I131 and I127 analyses in serum have been observed by Block et al. (3, 4) and by others (5), suggesting that the rate of iodotyrosine secretion from the thyroid is slower than that of iodothyronines. Further findings in support of A functional thyroid heterogeneity have been reported by Robbins et al. (6), who divided the thyroid fractions into a soluble and an insoluble component. The iodinated compounds were synthesized in these fractions in varying amounts and ratios. Several conclusions reported by other authors (7–10) could similarly admit of a plausible explanation on the basis of a thyroid heterogeneity concept.