Thyroid uptake and distribution of 125I, concentrations of T4 and T3 in serum and thyroids and concentrations of TSH in serum and pituitaries were measured to determine thyroid function of Uinta Ground Squirrels (Citellus armatus) native to 5,400 and 9,000 feet elevations to evaluate alterations in the state of the thyroid due to increased altitude. Uptake of 125I was 60 percent lower by squirrels from 9,000 feet. Incorporation of 125I into T3 was greater at the higher elevation, but absolute thyroid T3 concentrations were similar at both elevations. No significant differences were found in serum T3 concentrations, but squirrels living at 9,000 feet had higher serum T4 levels than those living at a lower elevation. Urinary excretion of T3 by squirrels from the 9,000 feet elevation was lower compared to excretion by squirrels native to 5,400 feet. Elevated pituitary TSH levels were also found in squirrels native to the higher altitude. These findings indicate a chronic hypothyroid state of squirrels living at 9,000 feet compared to those living at 5,400 feet.