Plasma growth hormone and insulin levels were measured in normal, thyroidectomized (TX), and TX rats treated with 1-thyroxine (1-T4) or rat-growth hormone (r-GH). Although normal growth was observed 5 days after surgery, a decrease of circulating levels of GH was evidenced as early as 5 days after the operation. However, the plasma insulin levels were not affected by the 5th, 10th and 15th day of thyroidectomy. With a more prolonged thyroid hormone deprivation, the plasma insulin levels of TX rats remained lower than those of age-paired controls. Treatment of hypothyroid rats with 0.05 micron 1-T4 for 10 days induced growth and an increase in circulating GH levels, but had no effect on the low levels of plasma insulin of TX rats. Treatment of TX animals with 0.10 or 0.20 micron 1-T4, or 100 micron r-GH during 10 days caused plasma insulin levels to shift toward the values of their weight-paired controls. However, treatment of hypophysectomized (HX) rats with 0.20 micron T4 during 8 days did not change the circulating levels of plasma insulin. It is suggested that thyroid hormone deprivation results in a decrease of circulating levels of insulin secondary to a deficiency in growth hormone secretion. In addition, normal GH secretion appears to be required for normal pancreatic insulin secretion to occur.