Discrete midline hypothalamic lesions were made in male rats in the region of the paraventricular nuclei (PVN), ventromedial nuclei, and medial preoptic area (mPO) using modified Halasz C-shaped knives. In euthyroid rats, small lesions, including the PVN and little surrounding tissue, or large lesions, including portions of the dorsomedial nucleus, anterior hypothalamus, and preoptic area in addition to the PVN, caused a similar 60% drop in the plasma TSH concentration within 2 days which persisted for at least 3 weeks. PVN lesions also produced a significant decrease in plasma TSH in hypothyroid rats and diminished both the increase in plasma TSH in response to thyroidectomy and the decrease induced by ether inhalation. Ventromedial nuclei lesions preserving the PVN inconsistently decreased plasma TSH. mPO lesions anterior to the PVN induced a transient elevation of plasma TSH and GH only in hypothyroid rats. TRH-stimulated TSH secretion was not affected by any of the lesions. The results suggest: 1) the PVN and their immediate vicinity are of primary importance for maintaining a normal TSH response to the stimuli investigated, and 2) the mPO area tonically inhibits TSH secretion, presumably through its role in somatostatin secretion.