The effects of various peptides on the thyroid were studied in mice injected with 131I and pretreated with thyroxine. Lysine or arginine vasopressin led to an increase in blood levels of 131I 0. Oxytocin and Val-5- angiotensin II were inactive. The greatest effects on blood levels of 131I were elicited with corticotropin A and natural or synthetic α- and β-melanocyte stimulating hormone (MSH). α- and β-MSH also increased the uptake of 131I in mice. Natural and synthetic aand β-MSH were active in hypophysectomized mice, showing that they might act directly on the thyroid gland. The MSH hormones had little if any effect on the uptake of 131I in tadpoles. (Endocrinology74: 559,1964)