Relatively little is known of the exact role of the thyroid gland in teleosts. Although the results of a few studies have suggested that the thyroid influences respiratory metabolism, as in the higher vertebrates, a greater number of experiments have failed to show that either treatment of fish with thyroid hormones or inactivation of the gland produces any significant effect upon the oxygen consumption of the fish. Thyroid activity has been linked with reproduction in some fish, although there is little evidence to show that the relation-ship is more than coincidental. The thyroid has also been connected with osmoregulation, but reviewing this work Pickford (1957) concludes that the relation with osmoregulation is only a secondary or collateral one. Recently Hoar (1953, 1955) has suggested that the thyroid may control the migratory response of fish. However, in the migratory species which have been studied in detail, the salmon, Salmo salar L. and Onchorhynchus spp. and the eel, Anguilla vulgaris, changes in the salinity of the environment during their migrations were complicating factors since salinity changes alone have been shown to stimulate thyroid activity in non-migratory fish (Olivereau, 1954).