Abstract Chronically implanted aortic and jugular cannulae were used to study thyroid status in the ground squirrel by a body pool turnover technique. Thyroid secretion rates (TSR) increased after cold exposure, and then began to decline with time in the cold until hibernation. Protein-bound iodine values were unchanged from controls through cold exposure and into hibernation. Binding of l-T 3− 125I by serum thyroxine-binding proteins increased after cold exposure and was twice the control values during hibernation. It is believed that alterations in thyroxine metabolism begin before hibernation is entered, hence reduction of body temperature is not the only factor involved in reduced thyroid activity.