1. The oxygen consumption of beef thyroid slices in Krebs-Ringer phosphate solution containing carrier free Nal¹³¹ was measured. After incubation, thyroid slices were digested and iodine fractions of hydrolysate were determined by paperchromatography.
2. Estradiol suppressed oxygen consumption, inorganic I¹³¹ uptake and organic binding of iodide of thyroid slices at the higher concentration, 10^-4M. While, at the lower concentration, around 10^-6M, the hormone slightly stimulated hormonogenesis.
3. Progesterone showed an inhibiting effect on hormonogenesis of thyroid slices at higher concentrations, 10^-3 and 10^-4M but not so profoundly as estradiol, when both effects were compared in same molar concentration. At the lower concentration, 10^-6M, the inhibiting effect was absent.
4. Cholesterol at the concentration of 10^-4M, used as a control substance of steroid compound, had no effect on hormonogenesis of thyroid slices.
5. Estradiol and progesterone at concentrations of 10^-4 and 10^-6M did not show significant effect on oxidative phosphorylation of thyroid slices.
6. From the data obtained in this study, the role of the hormones in the physiology of the thyroid gland is discussed in view of the results obtained in in vivo experiments.