In vitro studies have shown that the median eminence is capable of accumulating large quantities of thyroxine by an energy-dependent mechanism. Uptake is linear over a 20 min period and reaches equilibrium by 60 min at a distribution ratio of 60 (0.01 mM thyroxine starting concentration). Active k transport is markedly inhibited by lowered temperature and by treatment with ouabain (10–3 m), iodoacetate (10–3 m), sodium fluoride (10–2 m), and potassium cyanide (10–3 and 10–5 m). A large portion (60–70%) of the high intracellular concentration of thyroxine in median eminence is present in a pool readily available for release as unaltered hormone. Both uptake and release are influenced by physiological doses of TSH in the incubation medium; the former is stimulated and the latter inhibited by TSH. These data not only support the hypothesis that the median eminence is metabolically very active but also indicate that physiological control mechanisms for regulation of TSH secretion may reside in this tissue.