The effects on thyroid function of a new preparation of human thyrotrophin (hTSH) were studied in four subjects whose endogenous production of TSH had been suppressed by administration of thyroxine (T4). The hTSH, prepared from human cadaveric pituitary glands and highly purified using a monoclonal antibody technique, was given as an intravenous bolus of 2 i.u. hTSH. Serum TSH levels rose rapidly to a maximum of about 150 mu./l and then declined exponentially with a half-life of 100 min. After injection, the hTSH distributed rapidly in a volume averaging about 13 litres which corresponded approximately to the expected extracellular fluid volume of the subjects. Serum free tri-iodothyronine and free T4 rose significantly, reaching a maximum between 4 and 8 h after injection of hTSH; serum thyroglobulin was not altered significantly. The rise of thyroid pertechnetate uptake, a measure of the thyroid iodide uptake, occurred later, being only slightly increased at 8 h after administration of hTSH and reaching a maximum at 24 h.