The effects of cyaaide combined with protein deficiency on the thyroid have been studied in young iodine deficient rats Cyanide appeared to increase plasma TSH during protein deficiency more than in the protein-fed state. Acute administration of propylthiouracil decreased plasma thiocyanate. The significance of these observations is the presence of cyanogenic glycosides in the diet of many populations with endemic thyroid disease. In other studies the autonomy of the developing fetal thyroid system has been demonstrated. Projected investigations include studies on the effects of propylthiouracil on cyanide metabolism, the relative effects of linamarin, the cyanogenic glycoside of cassava, and of cyanide on the protein starved rat, the effects of CN minus administration on fetal development and thyroid function, and the effects of perturbation in the fetal thyroid system on later thyroid control.