Regulation of the pituitary-thyroid axis in rats with hypothalamic knife cuts has been studied. Complete hypothalamic deafferentation, either limited to the median eminence and the arcuate nucleus, or including parts of the dorsomedial nucleus and the whole ventromedial nucleus caused an increase in thyrotopin (TSH)-releasing hormone (TRH)-induced TSH secretion. Using an immunocytochemical procedure, a few TRH-positive fibers were observed within the median eminence of the larger island, while almost no fibers were identified in the smaller island. The exaggerated TSH response to TRH appeared within 3 days after the surgery and lasted for at least 1 week, when blood thyroxine (T4) level was significantly lowered. Exogenously injected T4 could inhibit such responses of TSH in the deafferented rats in a dose-related manner. These results support the hypothesis that the increase in the TSH response to TRH following hypothalamic deafferentation is due, at least in part, to the lowered thyroid hormone level in the blood.