We explored whether endogenous TRH in the hypothalamus produced a potential change in its receptors in the rat anterior pituitary. Thyroidectomy caused a progressive increase in the pituitary TRH binding with a concomitant increase in the blood TSH level. Injection of T4 prevented the increase in the pituitary TRH binding and the blood TSH level after thyroidectomy. The hypothalamic deafferentation and electrolytic lesions in the hypothalamic paraventricular nuclei led to a significant reduction in both the content of TRH in the hypothalamic median eminence and the blood TSH level, but they affected neither the number nor affinity constant of the pituitary TRH receptors in thyroidectomized rats. The present data provide evidence that TRH receptors in the anterior pituitary are profoundly regulated by thyroid hormones, but not significantly by TRH in the hypothalamus in the rat.