UNTIL recently, the common belief was that, in fish, the thyroid hormone did not have a metabolic effect similar to the one observed in mammals, or even that it had no effect at all. Thus, Etkin and his collaborators were unable to detect any effect of thyroid extract or thyroxine administration on the oxygen consumption of the goldfish, Carassius auratus, and toadfish, Opsanus tau (Root and Etkin, 1937; Etkin et at., 1940). Hasler and Meyer (1942) experimenting with the goldfish, Carassius auratus, also obtained negative results with thyroxine. However, an increase in the oxygen consumption of the parrotfish, Bathystoma, was observed after treatment with parrotfish thyroid extract by Smith and Matthews (1948), but only in the larger animals. Krokert (1936) observed changes in the shape of the fins of guppies, Lebistes reticulatus, fed thyroid extracts. Smith and Everett (1943) were unable to reproduce these results, while Hopper (1950, 1952) found that thyroid treatment accelerates gonopod differentiation in this species.