Bilateral injections of tri-iodothyronine (T3, 2 ng in 2 microliter artificial cerebrospinal fluid, CSF) were made into the hypothalami of 15 hypothyroid rhesus monkeys (Macaca mulatta) prepared with chronically implanted intrahypothalamic cannulae. In 29 out of 96 such injections, the concentration of thyrotrophin (TSH) in the plasma fell rapidly by more than 30% and returned to the basal value over the succeeding 48 h, in 13 experiments the fall was 20-30% and in the remaining 54 experiments the level of TSH was unaltered. With [125I]T3, the extent of diffusion of T3 through the hypothalamus in these experiments was shown to be very limited. The hypothalamic injection sites were subsequently identified histologically and it was found that in those experiments in which the level of TSH in the plasma had fallen, T3 had been injected into either the dorsomedial nucleus or the lateral hypothalamic area extending into the preoptic region. Injection of artificial CSF alone into these areas of the hypothalamus did not affect the concentration of TSH. Direct intrapituitary injections of T3 (4 ng) resulted in small and inconstant changes in the concentration of TSH in the plasma and injections of T3 (400 ng) into the third ventricle were without any effect. These experiments demonstrate that T3 can rapidly inhibit the secretion of TSH by a direct action upon defined parts of the hypothalamus.