ALTHOUGH the reptile has received but little attention in studies of L thyroid physiology, it has been shown that the reptile thyroid gland 1) contains iodine (1), 2) contains a substance similar to thyroxine (2), 3) gives a typical histological response to thiouracil and thyrotropin (3, 4, 5, 6). However, the functional significance of thyroid hormone in reptiles remains in doubt (7). It is the purpose of the present report to present data on the metabolism of radioiodine by the turtle. MATERIALS AND METHODS Two species of adult turtles were utilized, a terrestrial species, Terrapene Carolina, and an aquatic species, Pseudemys floridanus. The Terrapene were collected in the vicinity of Brookhaven National Laboratory during the months of June and July, while the Pseudemys were purchased from a commercial source and it was not certain that they were truly representative of the season in which the)’ arrived at the laboratory.