THE physiological role of the thyroid gland in the fishes remains largely enigmatic, since most research has demonstrated a lack of response of oxygen consumption to thyroidal hormone (Root and Etkin, 1937; Hasler and Meyer, 1942; Smith and Everett, 1943) in this group of animals. It is possible that the piscine thyroid is involved in salt metabolism and osmoregulation (Koch and Heuts, 1942; Fontaine and Koch, 1950), and there is some suggestion that thyroxine may play a role in certain specialized phases of maturation or “metamorphosis” in fishes (Sklower, 1928; Harms, 1934). Indeed, it has been established that the pigmentary changes, for example, in transformation of the young “parr” trout to the “silvery smolt” form are influenced by thyroxine treatment (Landgrebe 1941; Robertson, 1949). On the other hand, Stokes (1939) has shown that thyroxine will not initiate metamorphic changes in the larval lamprey. Using thyroidal material derived from the Parrot fish, Smith and Matthews (1948) were able to demonstrate some stimulation of oxygen consumption in another fish, the white grunt, Bathystoma.