The in vitro utilization of l-14C- and 6-14C-glucose was studied in the neuroendocrine structures of the hypophysis and hypothalamus 2 months after thyroidectomy in rats fed a low iodine diet. At this time, their PBI was markedly decreased and TSH levels were increased. Elevated levels of 6-14C-glucose uptake and its conversion to 14CO2 and 14C-lactate were obtained in the posterior hypothalamus from the thyroidectomized rats compared to control animals. The formation of 14CO2 from 1-14C-glucose was increased in both the median and posterior hypothalamus of the thyroidectomized animals. No differences were found between thyroidectomized rats and controls in the uptake of glucose and formation of 14CO2 and 14C-lactate in the hypophysis and in the anterior hypothalamus. The effects of thyroidectomy on glucose metabolism are discussed in terms of the functions of the neuroanatomical structures in the hypothalamus and hypophysis of the rat.