The hypothesis of an action of the pituitary gland of the developing chick embryo in the transfer of iodide from the yolk of the egg to the circulation of the embryo, through the yolk sac, was tested. Plasma iodide levels and thyroidal iodine contents were determined in hypophysectomized (by partial decapitation), thiourea-injected and control embryos. From day 11 of incubation these parameters were always lower in the 'hypophysectomized' embryos than in controls, and plasma iodide levels of the thiourea-treated embryos were higher than those of controls. These results indicate a reduced iodide transfer from the yolk to the 'hypophysectomized' embryo, and an increased iodide transfer to the thiourea-treated embryo. This occurred in spite of a reduced thyroid hormonal secretion in both series. The pituitary gland could therefore have a direct action (not through the thyroid gland) at the yolk sac level, to augment the transfer of iodide from the yolk in intact embryos from day 11 to the end of incubation. Thyroid-stimulating hormone (TSH) could be the pituitary hormone acting at the yolk sac level, the increased iodide transfer observed in the thiourea-injected embryos being due to a raised TSH secretion responding to the decreased plasma thyroxine levels.